“Sense data”, or “sense datum” in thesingular, is a technical term in philosophy that means “what isgiven to sense”. Sense data constitute what we, as perceivingsubjects, are directly aware of in perceptual experience, prior tocognitive acts such as inferring, judging, or affirming thatsuch-and-such objects or properties are present. In vision, sense dataare typically described as patches exhibiting colors and shapes. Forthe other senses, they would manifest sounds, tastes, odors, andtactile qualities. Suppose that you are looking at a brown table witha white coaster on it; your sense data would be a patch of browncorresponding to the brown expanse in your field of view, along with aroundish-shaped white patch. Based on such data, you might come toaffirm that a brown thing and a white thing, or a table and a coaster,are present before you.

Starting in the early twentieth century, the notion of “sensedata” played a central role in discussions of knowledge and ofthe ontology of perceptual experience. In epistemology, sense datawere offered as a basis for all other sensory knowledge. They wereepistemically privileged: many authors held that one could not bewrong about one’s sense data. Concerning what sense data are(their ontology), there has been a change of consensus. The earlyadvocates of sense data (explicitly so-called) mainly considered themto be mind-independent, that is, to exist apart from the mind, perhapsas a special kind of thing, neither mental nor physical. In the latterpart of the twentieth century, sense data came to be viewed most oftenas mind-dependent, as mental objects or contents of which we aredirectly aware in perceptual experience. Sense data dominateddiscussion of perception in the first half of the twentieth century.The notion is now frequently used as a stalking horse in thephilosophy of perception, as an example of a position that should beavoided.

This entry first examines the classical meaning of the term“sense data” and the concept of a sensory given. It thenpursues the history of these notions, from the early modern period tothe mid twentieth century, before turning to major critical responsesthat led to a decline in the popularity of sense data. Finally, itconsiders recent arguments for sense data and concludes by placingsense-data theory within a larger genus of perceptual theories thatdistinguish having sense impressions from the perception of individualobjects.

• 1. The Classical Notion of Sense Data: Term and Concept
• 2. Historical
  • 2.1 The Concept of Sense Data in the Modern Period
    • 2.1.1 Sense Data and Early Modern Philosophy
    • 2.1.2 Hamilton, Mill, and Sense Data
  • 2.2 Formation and Flourishing of the Concept and Term “Sense Data”
    • 2.2.1 Moore and Sense Data
    • 2.2.2 Russell and Sense Data
    • 2.2.3 Further Development of the Classical Notion of Sense Data
• 3. Major Critical Responses or Oppositions
• 4. Sense Data Recently
• 5. Conclusion
• Bibliography
• Academic Tools
• Other Internet Resources
• Related Entries

1. The Classical Notion of Sense Data: Term and Concept

Consider your present visual experience. Suppose that you are seatedat a table in a garden. You are looking at objects on the table and atthe vivid colors of flowers in nearby beds. Now focus on a particularstatic instance of your looking: the moment in which you see a whitecoaster on a brown garden table with a cut daisy lying just beyond thecoaster. The sense data for this scene would be the particular shapesand colors as immediately experienced, e.g., a white patch as seen outin front of you. You are looking at the coaster from an angle of aboutforty-five degrees. At this angle, many observers report the momentaryvisual experience of an elliptical white shape. (Try to experiencethis for yourself by observing a round coaster on a table at thesuggested angle. As we will discover, not everyone agrees that thepatch looks elliptical: some find a circle at a slant.) The table topis small enough that you can take it in at one glance: at this angle,many describe their momentary visual experience as containing atrapezoidal brown expanse, with the larger base of the trapezoidtoward them. The daisy presents sense data of a specific yellow andround shape, white petal-shapes, and green leaf- and stem-shapes.

These descriptions of sense data focus on the basic visual propertiescolor and shape: the facts that we are seeing a table, a coaster, anda daisy are not, in the usual sense-data theories, part of what isgiven. Typically, the sense data are just patches with a shape and acolor; thus, if the daisy were partly obscured by a mug that had beenplaced on the coaster, there would be two green shapes among the sensedata, on either side of the mug-shape, with no notion that the onebelonged with the other. Viewers typically are, of course, aware thatspecific objects are present. In that case, the theory says that theyhave gone beyond their sense data to perceive objects or things ofvarious kinds (table, coaster, flower), as opposed to the bare shapeand color of each sense datum.

The technical term “sense data” was made prominent inphilosophy during the early decades of the twentieth century by G. E.Moore and Bertrand Russell, followed by intense elaboration andmodification of the concept by C. D. Broad,^[1] H. H. Price, and A. J. Ayer, among others. Although the promoters ofsense data disagreed in various ways, they mainly agreed on thefollowing points:

1. In perceiving, we are directly and immediately aware of asense datum.
2. This awareness occurs by a relation of direct mentalacquaintance with a datum.
3. Sense data have the properties that they appear tohave.
4. These properties are determinate; in vision, weexperience determinate shapes, sizes, and colors.
5. Our awareness of such properties of sense data does notinvolve the affirmation or conception of any object beyond thedatum.
6. These properties are known to us with certainty (andperhaps infallibly).
7. Sense data are private; a datum is apprehended by onlyone person.
8. Sense data are distinct from the act of sensing, or theact by which we are aware of them.

This list constitutes a basic analysis of the Classical Notion ofsense data.Item (1) says that sense data are not perceived by the mediation of anythingelse. This was often put by saying that sense data are the immediateobjects of acts of sensing.Item (2) states that our mental relation to sense data is one of directacquaintance.(3) says that if we immediately sense a brown patch of a certain shape,we are acquainted with an item that has the phenomenal brownness andthe shape as present in our experience. That is, however color may beconsidered in physical science (as a microphysical property of objectsurfaces, for instance), the sense datum possesses the phenomenallypresent color property.(4) indicates that such properties are experienced as determinate; forexample, we experience a specific shade of brown.Item (5) distinguishes the awareness of sensory qualities such as color andshape from the judgment or affirmation that a material object (orother entity) might underlie the sense datum.(6) claims that each perceiver is certain about, and, in a strongerversion, cannot err about, there being an instance of brown color of acertain shape that is experienced.(7) holds that sense data are private, or perhaps that they inhabit aprivate space, so that sense data can be related to one another forone perceiver, but the sense data of one perceiver are not in animmediate relation to the sense data of another.(8) says that sense data are distinct from the mental acts by which weperceive them; in the Classical Notion, this typically included thatthey are not mind-dependent but are independent items.

It may be helpful to mention several things that are not included inthe above. Sense data are described as immediate objects of awareness,but there is no statement that we therefore only immediately perceivesense data and do not immediately perceive external objects, aconclusion which many theorists accepted. Further, there is nospecification of whether sense data are two dimensional or containdepth. Early theorists are typically read as asserting two-dimensional data;^[2] the second wave, including Price (1932: 3, 218–220), explicitlygave some of them depth. There is no statement about whether sensedata persist beyond the momentary sensing of them, as there wasdisagreement on this point. There is no assertion of their origin,causal or otherwise. It is natural to think of sense data as caused byobjects and as being related to those objects as appearance toreality, a position expressed by some authors (e.g., Russell 1912: ch.1; Broad 1923: 237–240). But that conception is not includedhere in the agreed-upon attributes of sense data in their earlyhistory. Similarly,item 5 distinguishes the apprehension of a sense datum from that of amaterial object or other entity; it takes no stand on whether suchobjects are inferred from sense data, are affirmed by a cognitive actthat is not an inference, or are treated in some other way. Nor doesitem 5 say whether sense data represent objects or their properties,independently of a further act of judgment or affirmation; someclassical theorists denied that sense data are intrinsciallyrepresentational. Further, sense data were not tied to specificneurophysiological processes, even if it was admitted by some authorsthat they are conditioned by neurophysiological states (such as thosethat produce afterimages). Finally, naïve realists sometimesdescribed the immediate apprehension of a portion of the surface of anobject as a perceptual datum. Present-day taxonomies of philosophiesof perception typically contrast naïve direct realism withsense-data theory, an outlook that results in part from the presenttendency to treat sense data as mental (Crane & French 2015[2021]). But in the classical period, there were arguments to theeffect that sense data are direct perceptions of a part of the surfaceof a material object (Moore 1918–19: 23–24), a positionthat Price (1932: chs. 2–3) described and sought toundermine.

In the above statement of the Classical Notion, nothing explicit issaid about the metaphysical classification of sense data. They mightbe events, substances, states of substances; they might be eitherphysical or mental. As mentioned, the majority of authors involved inthe original discussion held that sense data are not mental. But sometheorists advised that the term “sense data” in its basicmeaning should be treated as neutral on ontology (e.g., Moore1913–14; Price 1932: 18–20), even if further argument ledto a view that they are not mental. For the neutralists, acceptingthat sense data exist should be compatible with various theories ofperceptual ontology, such as naïve realism, representativerealism, Berkeleyan idealism, and Kantian transcendental idealism.

Early theorists who considered sense data to be mind-independenttypically thought of them as persisting through time. Russell, inearly sense-data writings (1912: ch. 1), viewed such data as existingapart from the mind as a special kind of thing (neither mental norphysical), which was commonly designated as atertium quid or“third thing”, in addition to objects (such as a physicaltable) and the perceiver’s mental states. Such intermediarythird things might be epistemically given only in the act of sensingthem, but they would not depend for their existence on that act. Thisled to the notion of unsensed sense data (e.g., mind-independentpatches of color), which were sometimes called“sensibilia” to indicate that they could be sensed ifsomeone were at the right location, but that they existed in any case(Russell 1914b: sec. 3). In the second half of the twentieth century,when sense data came to be viewed mainly as mind-dependent, they wereconsidered to be mental objects or contents of which we are directlyaware. Accordingly, the white shape of the coaster-shaped patch wouldbe a property of the mental object that is experienced, which wastaken to mean that the mental state itself would have among itsproperties a white and roundish patch of color (Jackson 1977: chs.4–5).

The initial positing of sense data arose within an extant discussionof sense perception and its objects. In Oxford and Cambridge of thelate nineteenth century, it was common to discuss perception as partof the “problem of the external world”, an epistemologicalproblem found in modern philosophy, from Descartes onward, concerningwhether we know an external world beyond our own minds and, if so, howwe know it. These discussions typically assumed a distinction betweensensation and perception, similar in many ways to that later drawnbetween sense data and the perception of material objects. A standarddescription of how vision occurs would say that sensations of shapeand color are first produced via the stimulation of the sensorynerves, and that from such sensations the perceiver forms perceptionsof objects, perhaps through a process of judgment or through learnedassociative connections among sensations. Let us turn, then, to thehistory of theories of sensation and perception, and the rise of sensedata.

2. Historical

The philosophical practice of separating what is immediately given tovision by way of sense impression from further psychological orlogical acts of perception stems from antiquity.^[3] In the optical tradition (where “optics” covered thecomplete science of vision, from physics and physiology topsychology), the medieval Islamic philosopher Ibn al-Haytham (ca.1030) distinguished what is found in “pure sensation”, theimmediate result of stimulating the sense organs, from what depends onrecognition, judgment, and reason (Optics, II.3.25). Arguably(Hatfield 2020: 200), he included light, color, and visual direction(a correlate of shape) among the things immediately sensed. Theoptical tradition as promulgated in the medieval European Latin Westprovided a framework for the theories of vision of Descartes andBerkeley (Hatfield & Epstein 1979: 371). The distinction between(a) a core of sensation and (b) perception of objects in the world asthe result of further psychological processes provided the predominantframework for theorizing about the senses into the twentieth century.^[4] The proximate background for the arguments about sense data justafter 1900 was set by engagement with early modern philosophy andnineteenth-century British philosophy.

2.1 The Concept of Sense Data in the Modern Period

In discussions of early modern philosophy in the second half of thetwentieth century, Descartes, Locke, Berkeley, and Hume were sometimesportrayed as sense-data theorists,^[5] especially when suggesting that they abetted skepticism about theexternal world. Indeed, the Scottish philosopher Thomas Reid diagnosedthe external-world skepticism found in Hume as arising from the theoryof ideas (initiated by Descartes and adopted by the others) beingtaken by Hume to what Reid considered to be its logical conclusion(Nichols & Yaffe 2014 [2016]). This interpretation was known toour early sense-data theorists through study of Reid himself or viaWilliam Hamilton’sLectures on Metaphysics (1861), inwhich Reid’s claims were discussed. In the latter half of thetwentieth century, Richard Rorty (1979: ch. 3) portrayed Descartes andLocke as engendering a veil of perception. He did not use the term“sense data” to describe the fabric of the veil by whichthese authors isolated perceivers, but he endorsed Reid’s chargethat they created a veil of ideas. This reading has been challenged(e.g., in Yolton 1984), and the present entry qualifies it in manyrespects.

2.1.1 Sense Data and Early Modern Philosophy

Several threads within early modern theories of perception provide abackground for the Classical Notion of sense data. First, there wasthe already-noted tendency to distinguish sensation from perception,or a passively received sensory core from a further perceptual act.For vision and touch, this further act typically included perceptionof objects as existing and as present in a locally coherentthree-dimensional space. The act might result from judgment,inference, or association. Second, the immediate objects of sensoryperception were sometimes rendered as mental representations fromwhich an external world must be inferred, possibly allowing forskepticism about the basis for such inferences. Third, these mentalrepresentations might or might not be attributed all or most of thefeatures of the Classical Notion. In particular, many early moderntheorists, as many theorists now, distinguished between representationvia intentional contents that do not literally possess the qualitiesthey present (such as color or shape), and representation throughentities that possess the properties (such as color or shape) as theyappear.

The terminological distinction between “sensation” and“perception”, explicitly so formulated, is usuallycredited to Reid, although it can be found in Nicolas Malebranche(Simmons 2009). Taken as a broad conceptual distinction with severalterminological and substantive variants, it is more widely present.Descartes, in theMeditations (1641 [1985: 55]), speaks ofthe perception of objects as arising from sensory ideas caused by suchobjects. Subsequently, in the Sixth Replies, he distinguishes betweenthe immediate mental products resulting from stimulation in the senseorgans, nerves, and brain, such as perception of an extended area oflight and color, and subsequent inferences or calculations that yieldperception of an object with a specific size, distance, and shape(1641 [1985: 295]). In other places, Descartes offers a physiologicalaccount of the perception of distance that renders the third dimensionpart of the sensory core (Dioptrics, 1637, Sixth Discourse[1965: 105–106]; see Hatfield 2020: 212–213); even so, ajudgment would still be needed for object perception.

Locke, in theEssay (1690: bk 2, ch. 9, art. 8),distinguishes between original sensory ideas and the perception of anobject as the result of an unnoticed act of judgment:

We are farther to consider concerning Perception, that theIdeaswe receive by sensation, are often in grown Peoplealter’d by theJudgment, without our takingnotice of it. When we set before our Eyes a round Globe, of anyuniform colour,v.g. Gold, Alabaster, or Jet, ’tiscertain, that theIdea thereby imprinted in our Mind, is of aflat Circle variously shadow’d, with several degrees of Lightand Brightness coming to our Eyes. But we having by use beenaccustomed to perceive, what kind of appearance convex Bodies are wontto make in us; what alterations are made in the reflections of Light,by the difference of the sensible Figures of Bodies, the Judgmentpresently, by an habitual custom, alters the Appearances into theirCauses: So that from that, which truly is variety of shadow or colour,collecting the Figure, it makes it pass for a mark of Figure, andframes to it self the perception of a convex Figure, and an uniformColour; when theIdea we receive from thence, is only a Plainvariously colour’d, as is evident in Painting.

The passage makes a clear distinction between the immediate impressioncaused by an object, in this case, sensation of a flat circlevariously colored, and the subsequent act by which, after a process oflearning, the faculty of judgment “frames … theperception of a convex Figure”. In a similar vein, Berkeleydistinguishes between the immediate objects of sight, which includelight and color forming a visible extension with visible figure andmagnitude, and the mediate objects of sight, informed by touch(Essay Towards a New Theory of Vision, 1709: sec. 49, 55).The immediate object of sight for an approaching object grows invisible extension. But, as the result of learning through the sense oftouch, the perceiver experiences the mediate object of sight, anobject of constant size. Hume offered a similar example: a table,considered as an external object, retains a constant size; yet oursensory image of it varies as we approach or recede from it(Enquiry, 1748: XII.I.9).

A full tally would reveal that the positions of these early modernfigures are consistent withitem 5 of the Classical Notion:

Our awareness of such properties of sense data does not involve theaffirmation or conception of any object beyond the datum,

when the terminology is adjusted to substitute “sensoryideas” or “immediate objects of perception” for“sense data”. They might also agree on the privacy ofsensations or sensory ideas. The authors would differ among themselveson some other points, such as whether there is an act-objectdistinction and whether sensory ideas are experienced by directacquaintance.

As regards external objects, Descartes and Locke were realists whoheld that representations mediate the perception of external objects.Accordingly, perceivers immediately experience sensory ideas thatpresent properties such as shape and color and, through an act ofjudgment, come to perceive external objects as having properties thatcause those sensory ideas.^[6] Berkeley and Hume sought to turn this position against them byarguing that it has skeptical consequences. They sought to avoid suchskepticism, by adopting idealism in Berkeley’s case, which makesreally existent sensory ideas the objects of perception, allegedlyavoiding skepticism, or adopting a common-sense realism about externalobjects, in Hume’s common-sense persona.

The fact that our early modern authors shared with the ClassicalNotion a version ofitem 5 does not make them sense-data theorists. Two key ingredients areitems 3 and 6: “Sense data have the properties that they appearto have”, and “These properties are known to us withcertainty”.

As regardsitem 3, current consensus is that it does not represent Descartes’position. He did not believe that states of mind such as sensory ideashave properties of color or other sensory qualities. Rather, he heldthat color and other qualities are in the mind“representatively”, or in terms of “objectivebeing” or “objective reality” (Meditations,1641 [1985: 28–29, 74–75, 113–114]). Descartes ishere understood to be invoking a notion of intentionality orintentional being.^[7] The phenomenal red that one experiences is not an instance of thepropertyred that is predicated of the mind, but rather theidea of red has an intentional content that is experienced asphenomenal red (Yolton 1984: ch. 1; 1996: 74–75; Nadler 2006:91–92). Similarly, the mind does not itself become square whenit experiences a square.

Regarding Locke, a perusal of theEssay, bk 2, ch. 8, revealsthat he is not clear on the matter. Interpreters find two notions of“sensory ideas” in Locke: as objects of perception thatmight potentially instantiate color and shape, and in that way be likesense data; and as acts of perceiving that might exhibit color andshape only by way of representation (Yolton 1984: ch. 3), understoodhere to mean nonconceptual intentional content (seenote 7).

As has been mentioned, Berkeley is sometimes classified as an earlysense-data theorist. The issue is complex. On the one hand, he claimsto be a champion of common sense who “believes [his]senses”, that is, who holds that “the real things arethose very things I see and feel” and that “colours andother sensible qualities are on the objects” (ThreeDialogues, 1713: Third Dialogue [2008: 210]). Berkeley holds thatwe are immediately aware only of sensory ideas, and passages such asthese might suggest that those ideas bear property-instances of red orother qualities. On the other hand, he speaks of light and color as“passions or sensations in the soul” (ThreeDialogues, First Dialogue [2008: 178]), describes figures andcolors as “sensible appearances”, and equates them withsensory “ideas” (Three Dialogues, First Dialogue[2008: 185]). In thePrinciples (1710: pt 1, art. 72), hespeaks of God ordering our “sensations”, which he equateswith the “appearances of Nature”. He never speaks ofsensory ideas as being red, but only as being instances in which wesee red or have the sensation or appearance of red.

Berkeley directly addressed the question of whether sensory ideas havethe properties they exhibit in answering a challenge to his idealisticview that only immaterial minds and ideas exist. If minds areimmaterial and have no extension, how can we have an idea of anextended thing, for instance, a square? Here Berkeley in effectinvoked the Cartesian notion of objective reality or (in present-dayterms) intentional content. When challenged with the claim that

if extension and figure exist only in the mind, it follows that themind is extended and figured; since extension is a mode orattribute,

he answered:

those qualities are in the mind only as they are perceived by it, thatis, not by way ofmode orattribute, but only by wayofidea; and it no more follows, that the soul or mind isextended because extension exists in it alone, than it does that it isred or blue, because these colours are on all hands acknowledged toexist in it. (Principles, pt 1, art. 49; see alsoThreeDialogues, Third Dialogue [2008: 229–230])

Unlike sense data, the sensory ideas of an extended shape or a colordo not have the shape or color as a property; rather, perceived shapesand colors are a kind of intentional content, a way in which things appear.^[8]

As regardsitem 6, that perceivers enjoy certainty in the apprehension of sensory ideas,a standard view has Descartes and Locke asserting or seeking suchcertainty (Rorty 1979: chs. 1, 3). This reading has been challengedfor Descartes, Locke, and Berkeley (Yolton 1984: ch. 5; Hatfield2011). Accordingly, the real hope for a prominent early modernconception that approaches the Classical Notion, especially as regardsitems 3 and 6, lies with Hume.

Hume held a version of the sense-datum theory, which treated such dataas mental or phenomenal items. Indeed, he provided what became thefirst standard argument for the existence of sense data, the Argumentfrom Perceptual Variation, which holds that, for things that webelieve to possess stable properties such as shape, size, or color,our experiences of those things vary with the perceiver’sphysiological state and spatial position; hence, we cannot equate theshape we perceive with the thing’s actual shape. In fact,Descartes (Meditations [1985: 56–61]), Locke(Essay, bk 2, chs. 8–9), and Berkeley(Principles, pt 1, art. 14) all produced arguments that notedthe variation of sensory ideas in relation to actual or purportedproperties of material objects, using these arguments to distinguishimmediately perceived sensory ideas and their intentional contentsfrom the latter objects. But it is Hume who clearly used the Argumentto support the Classical Notion of sense data.

InAn Enquiry Concerning Human Understanding (1748), Humeconsiders the common-sense view that we directly perceive materialobjects, such as a table. This sort of naïve realism is, Humesays,

destroyed by the slightest philosophy, which teaches us, that nothingcan ever be present to the mind but an image or perception, and thatthe senses are the only inlets, through which these images areconveyed. (Enquiry, XII.I.9)

He then argues:

The table, which we see, seems to diminish, as we move farther fromit: But the real table, which exists independent of us, suffers noalteration: It was, therefore, nothing but its image, which waspresent to the mind.

As Reid (Intellectual Powers, 1785: Essay 2, ch. 14) was topoint out, if we take “seems” here to refer to anappearance of the table itself, then there is no contradiction:appearances of a thing can vary even if the thing does not. But weshould treat “seems” as an instance of what Chisholm(1957: 43–44) calls its “epistemic” use. To say thata thing seems to diminish in size is to endorse that it does diminishin size. Accordingly, the phenomenal size reveals the actual size ofthe object of perception, and that object gets smaller. Hence, we mayformulate the argument as:

1. The table that I see diminishes in size as we move away fromit.
2. The real table does not change in size as we move away fromit.
3. Therefore, the table that I see is not the real table.

Hume’s own conclusion, quoted above, is that we perceive only animage of the table. This conclusion arises from the larger context inwhich our immediate experiences are sense impressions (a version ofitem 1). Because the object seen diminishes but the real table does not, oursensory experience does not acquaint us with the table but withsomething else, a mental or phenomenal impression.^[9] As a result, Hume held that we cannot know an external world ofmind-independent objects, such as the purported “realtable”, but must restrict our sensory knowledge to sequences ofsense impressions.

Thus far, we have not found explicit endorsement of items 3 and 6.These items are found most clearly in Hume’sTreatise ofHuman Nature (1739–40). He there says that “everyimpression”, including both internal and external, and henceincluding sense impressions, “appear, all of them, in their truecolours, as impressions or perceptions” (Treatise,I.4.2.7; again, Chisholm’s epistemic “appear”). But,further,

since all actions and sensations of the mind are known to us byconsciousness, they must necessarily appear in every particular whatthey are, and be what they appear.

Otherwise, we must suppose

that even where we are most intimately conscious, we might bemistaken.

Hume here endorses a version of what is otherwise known as the“transparency” of the mind: if our impressions have aproperty, that property must appear to us and hence be known to us,and if our impressions appear in a certain way, they are that way.This holds of extension:

the very idea of extension is copy’d from nothing but animpression, and consequently must perfectly agree to it. To say theidea of extension agrees to any thing, is to say it is extended.(Treatise, I.4.5.15)

An impression of a square, which we are conscious of as extended, isreally extended. Our consciousness of sensory properties, as thoseproperties are present to us phenomenally, reveals the properties asthey are. We can’t be mistaken about them. Items 3 and 6 areaffirmed by Hume. Indeed, substituting “impression”,“idea”, and “perception” for “sensedatum”, we might ascribe to Hume all of the items in theClassical Notion, save perhapsitem 8, the act-object relation. In reading theTreatise andEnquiry, one might be tempted to ascribe item 8 to Hume, buthe notoriously rejected such ascriptions in the Appendix to theTreatise.

The need for philosophers to address the problem of the external worldwas widely regarded as a legacy of early modern philosophy to thenineteenth and twentieth centuries. At Cambridge and Oxford into the1890s, discussions of the problem of the external world referencedHamilton’sLectures on Metaphysics (1861) and J. S.Mill’sAn Examination of Sir William Hamilton’sPhilosophy (1872). In that decade and the next, G. F. Stout andJames Ward at Cambridge discussed the problem as did Thomas Case andJohn Cook Wilson at Oxford (Nasim 2008, Hatfield 2013b).

2.1.2 Hamilton, Mill, and Sense Data

Hamilton was Professor of Logic and Metaphysics at Edinburgh, and inthat capacity regularly gave lectures in each subject area. In hisLectures on Metaphysics, he offered a taxonomy of positionson the problem of the immediate objects of perception. This taxonomydistinguished those who accepted or rejected what Hamilton consideredto be the fundamental “fact of consciousness” or“datum of consciousness” (1861, 2: 86). InHamilton’s view,

consciousness gives, as an ultimate fact, a primitive duality; aknowledge of the ego in relation and contrast to the non-ego; and aknowledge of the non-ego in relation and contrast to the ego. (1861,1: 292)

Further, it is “given” that ego and non-ego are mind andmatter and that these are co-equal. Hamilton’s preferredposition, which he called “Natural Realism” or“Natural Dualism”, affirmed the datum of consciousness andheld that some portion of the external world is given to the subjectin immediate perception. He argued that, in vision, the physicalretinal image is the immediately given material entity (1861, 2: 130,179–184). Accordingly, we have intuitive knowledge of atwo-dimensional image, which we are able to understand as a projectionof the three-dimensional physical world. This position would allowHamilton to reject Hume’s Argument from Perceptual Variation,because the “real object” of perception, the retinalimage, does vary as we approach or recede from the real table. On thisview, we don’t immediately perceive the real table, but we inferit from a real item of the external world that we do perceiveimmediately: the retinal image.

Hamilton described positions that emended or rejected his fundamentaldatum of consciousness. Some rejected the fundamental datum and heldthat in perception we are immediately aware of something that differsfrom the external world. Hamilton called this a“representative” theory (1861, 1: 295), or, moreimpressively, he named it “Hypothetical Dualism” or“Cosmothetic Idealism”. Holders of this position aresubdivided

into those who view in the object immediately perceived, atertiumquid different both from the external reality and from theconscious mind, and into those who identify this object with amodification of the mind itself. (1861, 1: 298)

Hamilton effectively invoked a notion of sense data, even if theclosest he came terminologically was to speak of the“datum” of consciousness. He accepted most tenets of theClassical Notion, but exceeded that notion in considering the datum tobe the physical retinal image.^[10] He further described positions opposed to his own that are close topositing a sense datum, either as atertium quid or as amental representation. He is not clear on whether these purportedrepresentative entities would be ascribed the properties they seem tohave (item 3), but his emphasis on the power of introspective or intuitiveconsciousness would move him in that direction.

On one way of reading his work, John Stuart Mill offered argumentsagainst some immediately given sense data before they had been sonamed. Mill did not deny Hamilton’s premise that whatever isimmediately and intuitively known in consciousness is known withcertainty (Examination, 126), but he contended that somestates of consciousness that seem intuitive, such as the allegedlyintuitive experience of a material world, are actually the result ofprevious psychological processes, which build up habits of mindthrough association. They would accordingly be complex products, notintuitively given primary data.

Mill framed his attack on Hamilton’s introspective method as adispute over the basis for knowing the external world. Hamiltonpurported to find material existence through a simple and ineliminable(or “necessary”) fact of consciousness. Mill replied,grouping Hamilton together with Reid, Kant, and others:

The test by which they all decide a belief to be a part of ourprimitive consciousness—an original intuition of themind—is the necessity of thinking it. Their proof that we mustalways, from the beginning, have had the belief, is the impossibilityof getting rid of it now. This argument, applied to any of thedisputed questions of philosophy, is doubly illegitimate: neither themajor nor the minor premise is admissible. For, in the first place,the very fact that the questions are disputed, disproves the allegedimpossibility. Those against whose dissent it is needful to defend thebelief which is affirmed to be necessary, are unmistakable examplesthat it is not necessary. It may be a necessary belief to those whothink it so; they may personally be quite incapable of not holding it.But even if this incapability extended to all mankind, it might merelybe the effect of a strong association. (Examination,143–145)

Finding that a belief has been entrenched through past experienceunmasks it as merely habit-based and not a product ofnecessity-bestowing intuition. What seems to be a primitive datummight actually result from habit. We’ll return to thisdistinction below, anent Russell’s logical and psychological“hard data”.

2.2 Formation and Flourishing of the Concept and Term “Sense Data”

Formation of the concept and the term “sense data” isstandardly attributed to G. E. Moore and Bertrand Russell.^[11] The background to this development includes not only the problem ofthe external world but also their early and joint commitment toBritish Idealism. The affirmation of sense data denied a central tenetof such idealism, that perception yields no knowledge because it isalways an incomplete grasping of the whole or the Absolute. As Russellput it, looking back from 1943:

Bradley argued that everything common sense believes in is mereappearance; we reverted to the opposite extreme, and thought thateverything is real that common sense, uninfluenced byphilosophy or theology, supposes real. With a sense of escaping fromprison, we allowed ourselves to think that grass is green, that thesun and stars would exist if no one was aware of them, and also thatthere is a pluralistic timeless world of Platonic ideas. (1951:12)

As we shall see, the realism that Russell affirms here is in somerespects peculiar, for in one version it includes a denial ofmind-independent material things, but without embracing idealism.

Moore first developed the concept of sense data. His conceptionprimarily focused on epistemology, whereas Russell combinedepistemology with various metaphysical schemes for building from sense data.^[12]

2.2.1 Moore and Sense Data

Moore broke with idealism in 1898 and his “Refutation ofIdealism” appeared in 1903. This paper did not introduce theconcept of sense data, but it formulated the distinction that made hima realist, which he maintained throughout his life, between the act ofsensation and its object. Moore contended that every case of sensationinvolves awareness of two distinct elements, one common to allsensations and the other differing among them. He distinguished thecommon element, which he called “consciousness”, from thediffering “objects” of sensation, which might be blue inone case, green in another. He did not claim to prove that the objectis what it seems to be: a material object at a distance, but heendorsed a naïve realism, according to which we are directlyaware of “the existence of a table in space” just as weare directly aware of the conscious element (1903: 453).

In December, 1909, Moore gave a paper at the Aristotelian Society inwhich he introduced the term “sense data”.^[13] The paper again divided “mental acts” from their objects,and it observed that we are aware of a difference between mental actand non-mental object: when we perceive one color and then another,the act of consciousness is of the same type, whereas the two colorsthat are objects of the act differ. Such acts of consciousness canthemselves differ in quality (one mental act may be a perceiving,another a willing, etc.), and many mental acts may be conjoined toform a complex mental entity. But such acts are distinct from theirobjects, even if those objects exist only when we perceive them, as,Moore reports, some people believe about colors. He then introducesthe term “sense data”:

By sense-data I understand a class of entities of which we are veryoften directly conscious, and with many of which we are extremelyfamiliar. They include the colours, of all sorts of different shades,which I actually see when I look about me; the sounds which I actuallyhear; the peculiar sort of entity of which I am directly consciouswhen I feel the pain of a toothache, and which I call “thepain”; and many others which I need not enumerate. But I wishalso to include among them those entities called “images”,of which I am directly conscious when I dream and often also whenawake; which resemble the former in respect of the fact that theyare colours, sounds, etc.; but which seem, as a rule, likerather faint copies of the colours, sounds, etc., actually seen orheard, and which, whether fainter or not, differ from them in respectof the fact that we should not say we actually saw or heard them, andthe fact that they are not, in the strictest sense of the words,“given by the senses”. All these entities I propose tocall sense-data. And in their case there is, of course, no questionwhether thereare such entities. The entities meant certainlyare, whether or not they be rightly described as“sensations”, “sense-presentations”,“sense-data”, etc. Here the only question can be, whetherthey are “mental”. (1909–10: 57)

Moore’s remarks are not in the spirit ofarguing thatsense data exist but ofdrawing our attention to theirobvious existence; his argument concerns whether they are mental.Moore observes (1909–10: 58) that philosophers such as Stoutregard sense data as mental because they fail to distinguish act fromobject, consciousness of blue from the blue of which we are conscious.Having made this distinction between act and object, he finds itmanifest that the object is non-mental (1909–10:58–60).

Moore intended the term “sense data” to provide a neutraldescription for whatever is the immediate object of our awareness andhence to be neutral on the problem of the external world. Over thenext half century, he considered various theories about how sense datamight relate to physical objects. In these discussions, he sometimesused “sensibles” as a broader term that includes unsensedsense-data (1913–14: 357–358). Without following thevarious shifts in his preferred theory, we can note that he consideredfive relations that sense data might have to physical objects or otheractual or potential causes (Moore 1913–14, 1918–19, 1925,1957). On one conception, physical objects might be regarded as

1. collections of sensed and unsensed sense data,independent of any conception of the causal source of such data.

Bringing in potential causes of sense data, these might be:

2. a spiritual entity, as in Berkeleyan idealism;
3. an unknown thing in itself; or
4. physical objects, as in Locke’s view ofrepresentative realism.

Finally, a sense datum might be

5. a part of the surface of a physical object.

Moore (1913–14: 376) identified (1) with Russell and Mill (e.g.,Russell 1914a, b) and allowed that it might be right. But he favoredpositions (4) and (5). In (4), sense data stand in a relation tophysical objects, called “relation R”, which turns out tobe a representational relation (Moore 1925, 1957). In (5), theperceiver is directly aware of a part of the surface of a physicalobject.

In discussing sense data, Moore introduced some facts that illustratethe relativity of perception, using coins (1913–14:371–372). With U.S. coins, a penny is physically smaller than aquarter and both are round. Moore and other observers report that whenlooking at the coins from a position other than directly above them,they appear more or less elliptical, becoming thin rectangles whenseen edge-on. Further, the quarter, seen at a sufficiently greaterdistance than the penny, appears smaller than it. Moore wanted toaccount for these purported facts that exhibit perceptual relativity,while preserving the common-sense beliefs that we really do, on someoccasions, see coins; that the coins really are round; that they havean inside and a lower side, even though these aren’t presentlyseen; that the quarter is really larger than the penny; and that thecoins exist when we aren’t looking at them.

Position (4), a form of representative realism,^[14] accounts for the perceptual relativity that makes us want todistinguish the immediate object of perception from the properties weascribe to physical objects (Perceptual Variation). The immediateobject is elliptical; the penny, round. Although accommodatingrelativity, this position may be challenged for making it difficult toknow the properties of the physical object, for the object is onceremoved from perception. But Moore (1913–14: 379–380) wasnot swayed by this challenge, generally aligning himself with commonsense beliefs about physical objects.

Position (5) equates the sense datum with a portion of the surface of the physicalobject. In order to account for perceptual relativity, Moore(1918–19: 23–24) now violates a basic tenet concerningsense data: that we perceive them as they are (item 3 of the Classical Notion). If the penny is round and we are directly aware of its physicalsurface, even when viewed at an angle, we are perceiving a roundthing. But, Moore holds, the round thing is perceived to“seem” elliptical. It is not perceived tobeelliptical, as in the other four positions, and as Moore himself hadpreviously assumed (1918–19: 23). Hence, in holding (5), onemust distinguish the actual properties of the directly seen surfacefrom the properties it seems to have (1918–19: 24). There is noactually elliptical datum; rather, the circular datum simply seems tobe elliptical. Moore did his best to uphold the direct realism of (5)and at times favored it. But in his final publication (1957), heendorsed(4) instead.

2.2.2 Russell and Sense Data

In 1911–1912, Russell returned to a project he had started soonafter the break from Idealism, an investigation of the concept ofmatter. A need to first secure the foundations of mathematics haddiverted him from this project (and resulted inPrincipia).Now, from 1912 on, he approached the analysis of matter from theperspectives of epistemology and ontology, investigating our knowledgeand conception of the fundamental constituents of the external world.The main events concerning sense data proper run from theProblemsof Philosophy (1912) to theAnalysis of Mind (1921),although the legacy of sense data extends further.

Russell’s initial discussions in this period made epistemologyprimary. Consider the opening sentence of his 1912 popular book,The Problems of Philosophy:

Is there any knowledge in the world which is so certain that noreasonable man could doubt it? (1912 [1997: 7])

The first positive instance he offers is knowledge of sense data.

Russell finds that when he looks at an object such as a table, hisperception of the table does not reveal the actual properties of thetable but rather a series of sensations that vary in what theypresent. The character of color sensations change as one moves arounda table, sometimes being brown, but in cases of reflected light (say,from a window), giving a sensation of white. The shape of the tableappears as a sequence of quadrilaterals; viewed from one end, itappears as a regular trapezoid, and the physically parallel sidesyield a sense experience of convergence. Russell argues for theexistence of sense data with a version of the Argument from PerceptualVariation, which, for shape, runs as follows (1912 [1997:10–11]):

1. In looking at a table, what we see varies in shape as wemove.
2. The real table does not vary in shape.
3. Hence, what we see is not the real table.

He offers similar arguments for color and felt properties such ashardness.

Russell gives the name “sense data” to

the things that are immediately known in sensation: such things ascolours, sounds, smells, hardnesses, roughnesses, and so on. (1912[1997: 12])

In further discussion, he affirms or implies the eight elements of theClassical Notion. He further suggests that sense data may exist onlywhen sensed (1912 [1997: 23]). Like Moore, he emphasizes theimportance of distinguishing between the mental act of sensation andthe object of that act (1912 [1997: 12]). The act is mental, theobject need not be. Like Moore as well, he suggests that those whobelieve that sense data must be mental have failed to distinguish actfrom object (1912 [1997: 41–43]).

Russell further asks whether the table is real and, if so, what itmight be (1912 [1997: 11, 19–25]). He answers that the tablemost likely is a real material object, rectangular and brown.Accordingly, he assigns to material objects the role of causing sensedata. Sense data then “represent” the objects that causethem (1912 [1997: 23]). This is not part of the base concept of sensedata but is the result of further analysis. He has posited sense dataas “third things”, standing between material objects andperceivers, and serving as the immediate objects of perception.

InProblems, Russell distinguished knowledge by acquaintance,which involves a direct relation to sense data, from knowledge bydescription or “knowledge that”. Although bareacquaintance makes us aware of the properties of sense data, it doesnot assert any truths or involve any judgments, hence does notconstitute “knowledge that” something is the case (1912[1997: 46]). We come to know basic facts about sense data by makingjudgments about them. In obtaining sense data of a brown table, weeasily come to know that we are seeing something brown, or,subsequently, a whitish sheen, or an appearance of one or anothertrapezoidal surface. This is what Russell calls “knowledge ofthings bydescription” (1912 [1997: 46]). Descriptionis only possible if we have access to universals, such as brownness,whiteness, being a surface, being a trapezoid, and so on. According toRussell, repeated sensations allow us to become acquainted withsensory universals:

When we see a white patch, we are acquainted, in the first instance,with the particular patch; but by seeing many white patches, we easilylearn to abstract the whiteness which they all have in common, and inlearning to do this we are learning to be acquainted with whiteness.(1912 [1997: 101])

Having acquaintance with universals such as whiteness, we can makejudgments that yield knowledge by description of the properties ofthings. These are judgments of fact about sense data, which establishtruths.

Going forward, Russell retained the view that sense data (or theircounterparts) provide epistemological bedrock. But within a year orso, inOur Knowledge of the External World (1914a) and“The Relation of Sense-data to Physics” (1914b), hechanged position on the metaphysics of sense data in relation tomaterial objects. He came to view such objects as logicalconstructions out of sense data. He found no need to assert theexistence of material objects, whether the substantial“thing” of common sense or the atoms and molecules ofphysics (1914b: 109, 114–115). Rather, he populated the worldwith minds and sense data and new entities termed“sensibilia”. Things and micro-physical particles becamelogical constructions, not inferred entities as in his representativerealism of 1912. This adjustment may be seen as an extension of hisrecognition that sense data are very well known. Treating materialobjects as mere logical constructions avoids a shaky inference fromwell-known data to the actual existance of physical things.

During this development, Russell retained and clarified his conceptionof occurrent sense data, including the eight points of the ClassicalNotion. He retained the distinction between mental act and non-mentalobject (1914a: 76; 1914b: 113). Sense data are non-mental, but theyexist as sense data only while they are present to the mind (1914a:64; 1914b: 110). He now identified two additional entities, inferredrather than constructed, that would figure into his account of theexternal world: other people’s sense data, inferred from theirtestimony and ultimately relying on analogy with one’s own mindto posit other minds; and

the “sensiblia” which would appear from places where therehappen to be no minds, and which I suppose to be real although theyare no one’s data. (1914b: 116)

Sensibilia, as postulated entities, are particulars that become sensedata when a perceiver enters into a relation of acquaintance with oneof them. Thus, there are many potential trapezoidal appearances as ofa table. When someone has a sequence of table-like experiences, someof those trapezoidal appearances are manifest as sense data. At otherperspectives, not occupied by any observer, there are unsensedparticulars. As Russell put it:

I shall give the namesensibilia to those objects which havethe same metaphysical and physical status as sense-data withoutnecessarily being data to any mind. (1914b: 110)

The notion of “object” here is not that of an ordinaryphysical object. Rather, as the objects of sensation, Russell posits abevy of fleeting particulars, the exact time course of which hedidn’t specify.

Russell’s analysis of our perception of an external worldadvanced in other ways. InProblems (1912: ch. 1), he hadnoted that we are so accustomed to judging the “real”shapes of things that we believe “that we actually see the realshapes” (1912 [1997: 10]), that is, we uncritically assume thatwe see the table as rectangular. But when we adopt the attitude neededto produce a drawing of the table, we may come to notice theappearances of various trapezoidal shapes. InOur Knowledge(1914a: ch. 3), Russell further analyzes this difference. His analysisdepends on the relation between what is logically and psychologicallyprimitive or else derivative. Logically primitive beliefs are thosethat are not logically derived via inference or construction. In orderto strengthen our knowledge, we seek to align these primitive beliefswith the psychologically primitive. While having an ordinaryperception of the table with its actual shape, we mistakenly believethat this experience, which has resulted from a learned psychologicalprocess, is psychologically primitive; accordingly, we also designatethis perception of the table as logically primitive. Scientificpsychology now offers a point that Russell had attributed to learningto draw: that what is actually psychologically primitive is theperspectival view of the table as trapezoidal (1914a: 68). Armed withthis psychological finding, we endeavor to make the trapezoidal datumlogically primitive; what is psychologically primitive is more secureand so provides us with “hard data” by comparison with the“soft data” of the common-sense belief about thetable’s shape (1914a: 69–70). As with Mill’sapparently but not actually intuitive beliefs, before epistemologicalanalysis many beliefs about sensory things are logically primitive,because not reached through logical derivation, and psychologicallyderivative, because caused by extra-logical psychological processessuch as association. Upon further consideration, we favor beliefs thatare both psychologically and logically primitive (1914a: 69–70)and so establish sense data as logically primitive.

Russell’s postulation of sensibilia allowed him to gain inmetaphysical regularity and economy. Sensibilia make it that theobjects of sense, color patches and like, do not simply go out ofexistence when unobserved. Rather, sensibilia persist and we samplethem when we occupy positions that present specifictable-perspectives. But, further, he economizes by replacing the“real” physical table with sense data and sensibilia atvarious locations. These are coordinated between the senses, and soinclude those as of approaching the table with our eye until our facetouches it, a perspective that connects with the felt location oftable (1914b: 119–120).

By 1920, Russell proposed even greater ontological economy by givingup mental acts and the experiencing subject as distinct from sensedata. This meant that the “data” were no longer given to adistinct subject and so were improperly named as “sensedata”; henceforth, he spoke of “momentaryparticulars” or “events”. He explicated this“neutral monism” in theAnalysis of Mind (1921:ch. 1). He saw himself as adopting James’s (1904) position,according to which both the self and the external world are composedof one neutral stuff—momentary particulars, characterized byperceptual qualities such as color and shape (for visual particulars).Accordingly, he discarded the act-object analysis of mind and senseperception, as there is no longer recognition of a mind that acts butonly of various sequences of momentary particulars. The sequence thatincludes all the experiences that I’ve had today (and before)constitutes my mind, another sequence of such particulars constitutesyour mind. Physical objects, such as a table, become a differentsequence of particulars, which are like sensibilia in givingperspectives on a table or a room that is composed of them. If noone’s sequence of particulars intersects with those of aparticular classroom, then that room and its contents consist of aseries of momentary particulars now occurring without anyone present.The material entities of physics are, as before, treated asconstructions from momentary particulars.^[15]

2.2.3 Further Development of the Classical Notion of Sense Data

Many other classical theorists accepted the view that sense data arenon-mental. Broad (1923) and Price (1932) were realists whoacknowledged both sense data and material objects. Broad explored a“critical scientific realism” by elaborating the notion ofsense data (or, in his terms, “sensa”) from Russell andothers. His position was in some ways similar to traditionalrepresentative realism, but Broad denied that sensa are themselvesmental states, in the sense of being dependent on the mind for theirexistence (1923: 265–266), and, further, he denied that physicalobjects are inferred from sensa. Rather, sensing sensa induces anunderived belief in mind-independent physical objects, a belief thatis not reached by inference and cannot be justified by inference(1923: 268). Broad agreed with Russell’s point that, in ordinaryperceptual consciousness, sense data are not manifest but may becomeso through an act of attention. He rejected the picture that we firstfocus on sensa and then explicitly infer objects and their propertiesfrom them; rather, we “pass automatically”, by a primitivefeature of our psychology, from sensa (perhaps unnoticed) to judgmentsabout objects and their properties (1923: 246–248).

Price’s book,Perception (1932), is an especiallyprobing account of sense data, their ontological status and cognitiverole. He begins by isolating a sense datum as what is given inperception such that we cannot doubt its existence: the extant bulgypatch of red that occurs when a tomato is viewed in good light cannotbe doubted, even if we can doubt whether we are seeing a tomato orindeed any physical object at all (1932: 3). The bulgy patch isdirectly present in consciousness, that is, is “given” asa datum by means of a relation of mental acquaintance. Price, as hasbeen mentioned, recommended that the term “sense data”should be considered, in its meaning, neutral on ontology. But he wenton to argue for a specific conception of what sense data are,including a specification of their cognitive status and their ontologyas atertium quid.

Against the “causal” theory of perception (effectively,representative realism), Price argues that the concept of cause isneither necessary nor sufficient for our belief in physical objects,that even in spatial perception only in some cases do sense dataresemble physical objects, and that in any case material objects arenot inferred from sense data. He further maintains that although sensedata are distinct from material objects (contra naïve realism)and their existence may require the existence of minds, they are notmental (1932: 117–127, 316–317). He elaborates notions ofperceptual acceptance and perceptual assurance to describe thenon-inferential cognitive acts through which we accept and believethat material objects exist. The belief in material objects consistsin (a) the construction of a “standard solid” from sensedata, which is a spatial structure that fits the outer shape of aphysical object in three dimensions, together with (b) belief in acausally efficacious “physical occupant” of this location(1932: ch. 8).^[16]

Price (1932: ch. 2, esp. pp. 27–33) gave an early formulation ofthe renowned Argument from Illusion as supporting sense data inopposition to naïve realism. In this argument, if the penny lookselliptical or the table trapezoidal, these are illusions, since thepenny is round and the table rectangular. Price’s version of theArgument may be formulated as follows:^[17]

1. In viewing a soccer ball in the distance, we areacquainted with a flat circle.
2. The surface of the soccer ball itself is spherical.
3. Hence, what we are acquainted with is not the surface ofthe soccer ball. Call the object of acquaintance a sense datum.
4. The sense datum is illusory: the ball is spherical, notflat.
5. In walking toward and drawing near to the soccer ball,we are now acquainted with a bulgy, hemispherical surface; ourperception is now veridical.
6. There is a continuous series of perceptions of theball’s shape as we walk, transitioning from flat tohemispherical.
7. Our perceptual consciousness of this series exhibits noqualitative, phenomenal breaks when transitioning.
8. If there is no phenomenal break then there is noqualitative difference in the veridical and illusory experiencesconsidered as experiences.
9. It is not plausible that qualitatively similarperceptions are of radically different objects, the actual surface ofthe ball in the near case and an illusory sense datum in the distantand in-between cases.
10. Since I am surely acquainted with a sense datum in thedistant case, I am acquainted with a sense datum throughout.
11. Hence, the direct object of perception is a sensedatum.

The argument relies on the indistinguishability of far and nearperceptual experiences, considered as experiences, to argue that,since the far experience does not directly present the object’ssurface and shape, the near experience also does not directly presentthat surface and shape. Price fortified the appeal to far and nearexperiences with the observation that experiences of the ball’sshape vary continuously with distance, which he found to tell againstthe claim that nearby experiences of the ball are direct while otherexperiences are mediated by sense data.^[18] In addition, Price extended the argument to include cases ofhallucination (1932: 28–29), which are explained by the havingof sense data that are indistinguishable from veridical perception.Appealing to the phenomenal indistinguishability of various perceptualexperiences provides support for sense data across all three cases:veridical perception, illusion, and hallucination.

In subsequent discussions, this argument received much attention (seeAyer 1940: ch. 1; Robinson 1994: ch. 2; for an overview, Smith 2002:ch. 1). In contrast, when Russell, Moore, Broad, and others spoke ofPerceptual Variation and the relativity of perception, they did nottreat the elliptical appearance of a penny as illusory. They simplyaffirmed that, from various positions, the penny looks elliptical eventhough it is actually round. Further, although they may have allegedthe phenomenal indistinguishability of illusory appearances, such asthe bent appearance of a stick partially submerged in water, whencompared with ordinary appearances, this was not the basic argumentfor sense data. Their basic argument came from Perceptual Variationand the lack of a match between the penny as it appears and the pennyas they believed it to be physically.

A. J. Ayer (1940), contra Price and others, argued that the existenceof sense data is not a factual question but a matter of linguisticconvention. He held that all theorists of perception might agree onthe phenomenal facts about seeing pennies but disagree on theimplications for the real shapes of pennies. One theorist holds that,as the observer changes position, the penny appears elliptical butstays round; the theorist endorses sense data (by Illusion). Anothertheorist agrees about the series of elliptical appearances with amoving observer but holds that, when the penny looks elliptical, ithas changed shape to become elliptical (Ayer 1940: 17–18).According to Ayer, the competing theories can’t bedifferentiated empirically (they agree on the elliptical appearances)and so they become recommendations about perceptual language. Ayer(1940: ch. 1) found that the Argument from Illusion supports choosingsense-data language. He developed the language of phenomenalism, inwhich statements about material objects are understood as statementsabout actual and possible sense data (1940: ch. 5). In his scheme,even though sense data do not exist unsensed (possible sense data aremerely hypothetical), they need not be classed as mental states (1940:76–78). Ayer (1940: 61–65) departed from the other maintheorists in rejecting an act-object analysis of sensations and theirobjects (item 8).

3. Major Critical Responses or Oppositions

The topic of sense data was heavily discussed in the middle decades ofthe twentieth century, and some notable criticisms were published.^[19]

Austin’s lectures in Oxford (from 1947 on), critical of sensedata, were published in 1962 asSense and Sensibilia. Theytook aim at the doctrine that

we never see or otherwise perceive (or ‘sense’), or anyhowwe never directly perceive or sense, material objects (or materialthings), but only sense-data. (1962: 2)

Austin accuses Ayer (1940), Price (1932), and others of embracing a“scholastic” view that analyzes a few poorly understoodwords and “half-studied ‘facts’”. By contrast,he promises to show

that our ordinary words are much subtler in their uses, and mark manymore distinctions, than philosophers have realized; and that the factsof perception, as discovered by, for instance, psychologists but alsoas noted by common mortals, are much more diverse and complicated thanhas been allowed for. (1962: 3)

Austin’s criticisms focus on language use, even while dissentingfrom Ayer that the issues are entirely terminological (Austin 1962:ch. 6). He focuses on the “argument from illusion” fromAyer (also citing Price), and complains that ordinary cases ofperceptual relativity and variation, such as the elliptical-lookingpenny, are not normally taken to be illusions. In fact, as we haveseen, many earlier sense-data theorists would agree. Austin has littleto say about the Argument from Perceptual Variation from the classicaldiscussions. He finds the penny example to be atypical because pennieshave stable, sharply bounded shapes whereas many objects, such ascats, do not. In considering the Argument from Illusion, Austin (1962:52) objects to the continuity premise, which we have just seen inPrice: that if, in one case, we explain a mismatch between object andperception by introducing a special entity such as a sense datum, wemust then accept that in phenomenally similar cases we can’thave direct, veridical perception. (The continuity premise remains indispute today, see Crane & French 2015 [2021].) More generally,sense-data theorists had argued that in describing ellipticalappearances gained from a penny, one adopts a special sense of“see” that (1) does not require that an ellipticalmaterial object exists, but (2) does affirm that an elliptical sensedatum exists (1962: ch. 3). Austin’s denial that somethingelliptical must be seen in such cases draws on ordinary language. ButPrice and others distinguished different senses of perception wordsadvisedly; Austin fails to engage their reasons more directly.Further, he does not consider theorists who say that we perceivematerial things as a result of sensing sense data (e.g., Broad 1923:248).

Another challenge to sense-data theory sought to reveal a“sense-datum fallacy” (Chisholm 1957: 151–152). Thecharge is that sense-data theorists make inferences like this:something appears elliptical, therefore what we see is an appearancethat is elliptical.^[20] The fallacy consists in simply assuming that if something appears acertain way (such as Price’s flat circle produced by a sphericalball), something must exist that is that way (a sense datum). Criticsthen argue that, in asserting that something appears elliptical (orflat), one might simply be describing the character of the appearance,not asserting that an item with an elliptical shape (or flat surface)is present. Afterall, we normally do accept that things need not be asthey appear. In response, it has been contended that sense-datatheorists do not simply move from “appears elliptical” to“is elliptical” (Smith 2002: 35–36). Rather, thepositing of sense data most commonly arises through acceptance of anact-object analysis of perception (item 8), which also affirms that the objects of perception are directlyapprehended (item 1) and that they are as they appear (item 3). The arguments from Variation and Illusion may then be used to contendthat we often experience perceptual objects whose properties aredistinct from the properties ascribed to material objects, as in thepenny appearing elliptical, and that from the qualitative similarityof illusions (or variant appearances) and alleged direct perceptions,we should conclude that we are always acquainted with sense data. Sucharguments may indeed be challenged, but there has been no simple andunwary move from appears such to is such.

Theorists who sought to avoid this alleged fallacy held that a pennymight appear elliptical in a phenomenally direct way without anelliptical item being present to the mind. These are sometimes knownas “appearance theories”. One such position is theadverbial analysis, which holds that phenomenal qualities inhere inthe mental activity of perceiving but without being present as objectsor as property-instances. Accordingly, we perceive the penny itself,but perceive it in a certain manner: roundly, or elliptically, andredly (Ducasse 1942; Chisholm 1957: part 3). To say that we areexperiencing the penny elliptically is to say that we enjoy anelliptical appearance without there being an actually elliptical itempresent to us; rather, the appearance is understood simply as ourmanner of sensing the penny, or as the way our sensory activity makesus experience the shape of the penny. A related sort of appearancetheory renders the elliptical appearance as a mental content, or anintentional state, with the contents elliptical and red (Barnes1944–1945). Accordingly, via intentional content, one canexperience an elliptical spatial structure without being acquaintedwith an actually elliptical object. The intentional contentinstantiates an appearance as a state of the perceiver, whoseperceptual experience thereby represents the world as being a certainway, for instance, as containing an elliptical shape. (Intentionalisttheories are found in Dretske 1995 and Tye 1995; an important earlierformulation is Anscombe 1965.) These two types of theories account forVariation and Illusion via adverbial or intentional contents that canalso account for veridical cases of perception. Indeed, these theoriescan allow that, in some instances (with moderate slant), we see apenny that is round by seeing it as being round and slanted relativeto the line of sight. Adverbialists would say that we see the pennyroundly; intentionalists, that we see the penny via mental contentthat represents it as being round. (On these philosophies ofperception, see Crane & French 2015 [2021].)

There is a tradition in philosophy and psychology that denies, onphenomenal grounds, the distinction between impoverished, typicallytwo-dimensional sensory contents or states and the perception ofobjects in three dimensions. One can see William James’saffirmation that sensations are intrinsically world-directed (1890, 2:1–3) and exhibit three-dimensional volume (1890, 2:135–136) as objecting in advance to the Classical Notion ofsense data. James deniesitem 5, that sense data are neutral about a world beyond them, in favor of aposition that both sensation and perception give us “animmediately present outward reality” (1890, 2: 2). The Gestaltpsychologists (Koffka 1935: chs. 1–3; Köhler 1929 [1947:chs. 1–3]) also adopted a position in which we are perceptuallygiven a world, not an impoverished content or object distinct from theworld. They made perception of the valence or functional values of theworld into an immediately given aspect of perceptual content. Theyalso denied that two-dimensional sensations or sense data are the corefrom which perceptions are developed: the world is given in three-dimensions.^[21] The Gestalt theorists drew attention to the phenomena of size andshape constancy. If two objects of the same size are presented at fiveand ten feet, the retinal image of the farther object will be half aslarge (in linear height) as the image of the nearby object; yetobservers report them to be phenomenally of the same size (or nearlyso). Similarly, a circle seen at a (moderate) slant projects anellipse on the retina but appears circular. These theorists contendedthat significant phenomenal differences in size between the twoobjects, and the appearance of an elliptical shape, are produced onlyunder special laboratory conditions or through trained acts ofattention, as might be found in those who are taught to draw (Koffka1935: 222–223; Köhler 1929 [1947: 71–74]).Subsequently, the perceptual psychologist James J. Gibson (1950)agreed with the Gestalt theorists in arguing that the two-dimensionalexperiences that are like sense data arise from acts of attention thatartificially alter our phenomenology away from the three-dimensionalvisual world that we normally perceive. Accordingly, thetwo-dimensional visual field is not an immediate object of perceptionor primitive constituent of perception that is uncovered by attention.Rather, it is constructed through a special effort of attention.Gibson held that, in everyday perception of a circle at a slant (say,your neighbor’s plate at a dinner party), the object looks likea circle at a slant and so looks circular, not elliptical (1950: chs.1, 9). In subsequent writings, Gibson (1966, 1979) developed thenotion that sensory systems are attuned for presenting to perceivers anavigable environment and its affordances for meeting theperceiver’s needs and goals.

Each of these three theoretical stances denies, on phenomenal grounds,the two-fold character of perceptual consciousness, or the divisioninto impoverished sensuous element and accompanying interpretation orobject-positing. Rather, sensory experience is intrinsicallyworld-presenting, spatially expansive (in three dimensions), andutility alerting.

Wilfrid Sellars sought to unmask the “myth” of a puregiven to which subjects can respond atomistically in a way that yieldsan epistemic given (a primitive basis for knowledge). According toSellars’s highly regarded “Empiricism and the Philosophyof Mind” (1956), the “classical” sense datumposition holds that “s is red” can be knownnon-inferentially. Classical sense-data philosophers

have taken givenness to be a fact which presupposes no learning, noforming of associations, no setting up of stimulus-responseconnections. (1956: §6)

They accept an “inconsistent triad”:

Who allegedly subscribed to this triad? Some of Sellars’sexamples draw from early modern philosophy; he effectively attributesa version of A and B to Locke, Berkeley, and Hume (1956: §28)but, it seems, not C. He criticizes Ayer’s linguistic analysis(1956: §§8–9) but suggests that it is aimed more atanalyzing object-talk into sense-data talk than at showing sense-datatalk to be epistemically basic (as in A). He briefly alludes to Broad(1956: §11) and Price (1956: §31), without ascribing thetriad to them. He also devotes considerable attention further on tothe role of an ostensive “given” within the“positivistic conception of science” (1956: §43), andit is tempting to surmise that his main target was that conception asput forward by Carnap, Reichenbach, and others. To assess thatdirection, we would need to interpret the positivist shift fromelementary experiences to protocol sentences as epistemically basic,which would take us far afield (but see Leitgeb & Carus 2020).

In relation to the triad, a prominent reading of Locke has him imputeepistemic authority to sensory ideas as giving knowledge of theircauses (Ayers 1991: vol. 1, ch. 18). Locke is attributed A and B butnot C. It is doubtful that Descartes, who distinguished sensing fromjudging, would endorse A. Among the classical sense-data theorists,many would deny A, distinguishing sensing and“acquaintance” from “knowing that”; sensingred may usually and non-inferentially lead to knowing that s is red,but not as a simple matter of entailment. Russell explicitly denied A;sensing may be unacquired but “knowing that” involvesjudging, not mere sensing, and the universals that enter judgments ofsensory quality (such as whiteness) are acquired, affirming C. Sellarshimself rejects A, complicates B, and affirms C, adopting (byassumption) a position of “psychological nominalism”(1956: §29), according to which kind-terms, including colorterms, and the ability to be aware of color instances are learned aspart of a linguistic and conceptual web of meaning (holism).

In his holism about color terms, Sellars raised an objection thatwould engage Russell’s conception of atomic statements aboutsense data as epistemically basic (Russell’s psychologically andlogically primitive hard data). Sellars (1956) constructs an elaboratemyth of his own, in which our behaviorist ancestors must learn totreat thoughts as internal states of one another and subsequently toself-report their own thoughts (1956: §§47–59). Heextends the story from the attribution of inner thoughts to oneanother to the positing of internal sensory impressions (1956:§§60–62). His view differs from Russell’sposition in making the ability to be aware of red sense experiencesdepend on the acquisition of a sophisticated web of meaning andtheory; this ability is not “given”. Nor, as Russell wouldhave it, is the universalred needed for knowing that s isred, “easily acquired” (1912 [1997: 101]). Accordingly,there are no easily acquired color predicates to describe the sensedata that ground knowledge. Sellars asserts that

instead of coming to have a concept of something because we havenoticed that sort of thing, to have the ability to notice a sort ofthing is already to have the concept of that sort of thing, and cannotaccount for it. (1956: §45)

Sellars claims the advantage that, through this social process ofacquisition, sense impressions lose “absolute privacy” andbecome objects of intersubjective knowledge (1956: §62), and herehe directly departs fromitem 7 in the Classical Notion. But he has also rejected the acquaintanceview of sense data as immediately present to the mind prior tolearning (item 2). Sellars’s holistic conception that the acquisition ofphenomenal concepts enables the ability to observe and self-report thepresence of sense particulars may offer better reasons for rejectingitem 2 of the Classical Notion than are found in his discussion of theinconsistent triad.

4. Sense Data Recently

In philosophies of mind and perception, sense data fell out of favorin the 1960s and 1970s. But there have been attempts to restore someversion of the theory. Often, but not always, these efforts sought tosustain mental sense data as the immediate objects of perception. Themost extensive efforts are by Frank Jackson, inPerception(1977), and Howard Robinson, in a book of the same title (1994).

Jackson set out to support the conclusion that the immediate objectsof perception are mental and that they represent physical objects. Hefirst argues that what we immediately perceive are sense data, whichare colored patches. His argument appeals to the fact that when weperceive an object, we are not in immediate contact with the wholeobject but only a portion of it, and that we are immediately aware ofthe color and shape of this portion. We see the object in virtue ofseeing its surface, and the basic form of seeing its surface is to seeits color and shape via sense data that represent that surface. The“in virtue of” relation does all the work here; Jacksondoes not draw on, and rejects, the arguments from Variation andIllusion (1977: chs. 1, 4, 7).

This conclusion does not say whether sense data are mental or physical(1977: 119). The argument that sense data are all mental proceeds forthe case of vision by seeking to establish that colors are mentalitems. The argument is indirect, from the conclusion that colors arenot physical. This conclusion, in turn, relies on the claim that coloris not a “scientific property”, that only scientificproperties exist in the external world and affect the eyes and brain,and that color then can only be a mental entity caused by neuralactivity (1977: ch. 5).

The argument relies on a conception of scientific or physicalproperties that is widespread in philosophy: that physics has no placefor phenomenal properties or perceptual experience. Accordingly,physics posits particles and forces, which may cause perceptualexperience in a suitable perceiver, but which themselves arecolorless. Jackson is certainly right that physics today does notaccept something akin to phenomenal color as a fundamental physicalproperty; rather, it accepts wavelengths and brain processes, andthese possess no such fundamental property. Prior to the evolution ofcolor-sentient beings, there presumably were no phenomenal experiencesof color, hence none to be counted among physical phenomena.Nonetheless, physics, especially in the science of optics, accepts the“subjectiveperception of color” (Nelson 2017:125) as an object of study,^[22] just as acoustics accepts the experience of pitch as a physicalphenomenon (Weld & Palmer 1925: 322). Arguably, phenomenalexperiences need not be reduced or even reducible to basic physicalprocesses in order to be counted among physical phenomena and henceamong items to be investigated in the hard sciences, includingphysics. Accordingly, color experience needn’t be decisivelycategorized as “non-physical”. Its ultimate ontology isleft open. It might be a mental content as in an intentional theory orother appearance theory.^[23] The color property might be analyzed relationally, as a surfaceproperty with the power to produce a color experience, whichneedn’t be a colored sense datum. The metaphysical status ofphenomenal experience need not be decided in order for the conditionsof such experience to be examined in the science of physics.

Robinson (1994) sets forth the most extensive case for sense data inrecent times. His book offers a history of sense-data theory, which hesees as deriving from Locke, Berkeley, and Hume. Robinson thusdisagrees with the history offered above as regards Berkeley andLocke. He also specifies a standard conception of sense data (1994:1–2): that such data possess the sensible qualiteis they appearto have, and that they are not intrinscally representational (they donot intrinsically refer beyond themselves; they have no intentionalcontent). He takes the Argument from Illusion to be the primaryclassical argument for sense data in the twentieth century (though notmuch invoked before). Most of the other arguments he examines arerelated to Variation or Illusion, or to Jackson’s argument fromscience about color.

Some of Robinson’s arguments are new to our discussion, such asthe Time-Lag Argument. It notes that the light reaching our eyes fromstars may be hundreds or thousands of years old and so the star mayhave ceased to exist. On the assumption that the object of veridicalperception exists, our perception cannot be of the actual star, andhence must be of another object. But all perception involves a causalprocess that unfolds in time; hence, on the assumption that perceptionmust be simultaneous with its object, we never perceive the actualproperties of the things that reflect or emit light (1994: ch. 3).This argument can be extended to a Causal Argument (1994: ch. 6) whichappeals to the fact that we can be made to seem to perceive objects bynon-standard causal processes, such as those that underliehallucination. In such cases, we don’t perceive an externalobject and so there must be another object that we perceive, havingthe properties that appear to us (invokingitem 3 of the Classical Notion). These cases could be phenomenally indistinguishable from normalperception. Hence, arguing that phenomenally indistinguishable itemsshould be classified as the same type of object (as in Price’sArgument from Illusion, Sec. 2.2.3, above), we should conclude that inthe normal case we also don’t directly perceive an externalobject.

In addition to making these various arguments for sense data, Robinson(1994: ch. 7) also raises objections against what he takes to be theprimary opponents: intentionalism and the adverbial theory. He furthersuggests that, if his arguments for sense data are sound, this findingwould support a phenomenalistic idealism over physical realism (1994:chs. 8–9). (On idealism, see Guyer & Horstmann 2015 [2021];on realism, Miller 2002 [2019].)

For those who would avoid both mental and non-mental perceptualintermediaries, the theory of “Naturalized Sense Data”(NSD) is of interest (Bermúdez 2000). It distinguishes betweenwhat is directly perceived and what is immediately perceived. In thisscheme, three-dimensional objects are directly perceived but notimmediately perceived. We immediately perceive the portion of thesurface of the object that is visible at any moment. Because weimmediately perceive a portion of the three-dimensional object, we areable to make ostensive reference to the three-dimensional objectitself and so, it is claimed, to perceive it directly. NSD assumesthat naïve realism holds that we immediately perceivethree-dimensional objects as a whole; NSD therefore rejects naïverealism, since according to NSD we immediately perceive onlysurface-portions (2000: 369–372). NSD objects to traditionalsense-data theory as placing an unnecessary intermediary between theperceiver and ostended objects (2000: 370–371).

There are two problems with this position. First, as Bermúdezobserves, if it is to avoid bringing in mentalistic factors, itrequires a physicalist account of color as a surface property, whichhas not been forthcoming (2000: 368, n. 14; 373). Second, if aphysical surface-portion provides the immediate object of perception,then it provides the phenomenal content of our perception. When Mooreendorsed surface-portions as immediate objects, he noted the problemthat if a surface of a round penny (seen obliquely) is the object ofperception, it should appear round. Since he held that it did not, hefelt compelled to say that the sense datum appears elliptical but isround, thereby sundering the immediacy by bringing a separate“appearance” into the mix. A defender of NSD might counterMoore’s response by saying that the penny in fact looks roundand slanted, so there is no disparity between physical surface-portionand phenomenal perception. But then Price’s objection arises.Price allowed for what is now called full shape constancy within a fewfeet of the observer: the round penny at a slant would appear round.But he noted that, beyond a few feet, constancy starts to break down.^[24] This creates a problem for NSD: within the framework set out here, itwould need to recognize “appearances” with content thatdiffers from the actual structure of a surface-portion. This violatesthe intent of NSD, which is to establish an account in which thephenomenal content of perceptual experience is provided by theproperties of the surface-portion itself, without needing to invokeappearances in addition to immediately perceived surface-portions.

5. Conclusion

The core notion of a sense datum is an immediately given, minimalperceptual object, consisting in the case of vision of a shaped patchof color. Often, but not always, this datum was considered to be twodimensional. It typically was contrasted with the perception of athree-dimensional material object of a particular kind and displayinga panoply of further properties. In the classical period (the firstseveral decades of the twentieth century), sense data were typicallytaken to be non-mental and mind-independent, and were often regardedas representing material objects. In some cases, sense data or theirkin were taken to be elemental constituents in the composition orbundling of sense particulars. These particulars did not refer tomaterial objects per se but, when conjoined, composed a perceptualworld (as in Russell 1914a and 1914b). More recently, sense data havebeen viewed as mental items (e.g., Jackson 1977), and perhaps asleading to idealism (Robinson 1994). In a few cases, sense data havebeen equated with portions of the surface of a material object (Moore1918–19: 23–24; Bermúdez 2000).

The theory of sense data is a species of a genera of positions inperception theory, which make a distinction between an impoverishedsense impression and the perception, affirmation, or judgment of afurther entity, usually a material object. Although sharing thisdistinction, other species in this genus differ from sense-data theoryin various ways, the most fundamental being that they do not posit anobject having the properties of color and shape as found in consciousperception (item 3). Some views render perceived properties as appearances. One type ofappearance theory holds that shape, color, and other perceivedobject-properties are intentional contents (intentionalism). Anothertype holds that experiences of shapes and colors are manners or modesof perceiving (adverbialism). Adherents of both types of theory rejectthe act-object analysis and so would reject item 8 of the ClassicalNotion; and, of course, they would also reject items 1 and 2. Inanother vein, some versions of naïve realism hold that weimmediately perceive the surface of an object from a point of view andin three dimensions, and then augment the basic perception to includeconceptualized object content (Noë 2004, Brewer 2011). Atpresent, these various positions enjoy more favor than do theoriesthat posit sense data as mental objects or as third things. Indeed,recent discussions in the philosophy of perception frequently usesense data as an example of a concept or an entity to be avoided(e.g., Fish 2010: ch. 2).

Theories that distinguish impoverished sense perception from objectperception and cognition have been challenged by what Firth(1949–50) called the “percept theory”, according towhich immediate visual experience is of a three-dimensional visualworld that manifests object-properties and possibilities for action.This is the tradition of James, the Gestalt psychologists, Gibson, andsome of the appearance theories (e.g., Smith 2002). Such theories havegrown in favor recently. But sense-data theories have also showndevelopment. Many sense-data theories recognize the immediate objectsof perception to be three-dimensional. This would permit a penny seenat a moderate slant to be described as appearing as a circle at aslant, not an ellipse. Finally, although treatment of color as aprimitive property that literally inheres in sense data (whether suchdata are considered to be surface portions, mental objects, or thirdthings) is not widely favored, it is also true that, metaphysically,there is no settled home for phenomenally experienced color. Theendeavor to account for the phenomenal characteristics of objects andtheir properties is ongoing.

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Acknowledgments

I am indebted to an anonymous referee for many helpful queries andsuggestions and to Louise Daoust, Elle Kirsch, Pen Maddy, HollyPittman, Tiina Rosenqvist, and Evan Sommers for raising many importantpoints.