The knowledge argument aims to establish that conscious experienceinvolves non-physical properties. It rests on the idea that someonewho has complete physical knowledge about another conscious beingmight yet lack knowledge about how it feels to have the experiences ofthat being. It is one of the most discussed arguments againstphysicalism.

• 1. History of the Underlying Ideas
• 2. The Basic Idea
• 3. Some Clarifications
  • 3.1 Two Versions of the Argument
  • 3.2 Physical and Non-physical
  • 3.3 Knowing what it is like
• 4. Objections
  • 4.1 Doubts about the Thought Experiment
  • 4.2 Complete Physical Knowledge without Knowledge of all the Physical Facts
  • 4.3 No Propositional Knowledge 1: the Ability Hypothesis
  • 4.4 Objections Against the Ability Hypothesis
  • 4.5 No Propositional Knowledge 2: The Acquaintance Hypothesis
  • 4.6 The New Knowledge/Old Fact View
  • 4.7 Variants of the New Knowledge/Old Fact View
  • 4.8. Objections Against the New Knowledge/Old Fact View
  • 4.9 The Knowledge Argument and Objectivism
• 5. The Dualist View About the Knowledge Argument
• 6. Further Arguments
• 7. Concluding Remark
• Bibliography
• Academic Tools
• Other Internet Resources
• Related Entries

1. History of the Underlying Ideas

The Knowledge Argument became the subject of intense philosophicaldiscussion following its canonical formulation by Frank Jackson(1982). However, there are numerous precursors of this argument in theliterature. Precursors of the Knowledge Argument typically involve atleast one of two strategies which are familiar from Jackson’sformulation. The first is to appeal to what Daniel Stoljar & YujinNagasawa term theknowledge intuition: the intuition that noamount of knowledge of the physical information or physical factsconcerning certain experiences can by itself suffice for knowledge ofwhat these experiences are like, i.e., knowledge of their qualitativecharacter or distinctive qualia (2004, 2–3). The second is tomake use of thought experiments which are similar to Jackson’sfamous example of Mary. These thought-experiments typically involve abeing who has complete knowledge of the physical information orphysical facts concerning certain experiences, but who (it is claimed)lacks knowledge of what those experiences are like.

As examples of the knowledge intuition, Stoljar & Nagasawa citepassages by Bertrand Russell (1998, 13–14) and by J.W. Dunne(1929). As Dunne puts it, quoting James Ward, physical descriptionscannot entail knowledge of what it is that “you immediatelyexperience when you look at a field poppy” (1929, 5). Around thesame time as Dunne was writing, C.D. Broad (1925) used athought-experiment as part of an argument against a mechanisticversion of physicalism. Broad argues that even if the mechanistictheory of chemistry were true there still would be a property ofammonia that a mathematical archangel endowed with unlimitedmathematical skills and “gifted with the further power ofperceiving the microscopic structure of atoms” could notpredict, namely its smell:

He [the archangel] would know exactly what the microscopic structureof ammonia must be; but he would be totally unable to predict that asubstance with this structure must smell as ammonia does when it getsinto the human nose. The utmost that he could predict on this subjectwould be that certain changes would take place in the mucous membrane,the olfactory nerves and so on. But he could not possibly know thattheses changes would be accompanied by the appearance of a smell ingeneral or of the peculiar smell of ammonia in particular, unlesssomeone told him so or he had smelled it for himself (1925, 71).^[1]

Under the title “The Cognitive Role of Acquaintance,” H.Feigl (1958) briefly discusses the epistemic limitations of a Martianwho studies human behavior but does not share human sentiments:

The first question I wish to discuss concerns the cognitive‘plus’, i.e., the alleged advantages of knowledge byacquaintance over knowledge by description. We may ask, for example,what does the seeing person know that the congenitally blind personcould not know. Or, to take two examples from Eddington, what could asomeone know about the effects of jokes if he had no sense of humor?Could a Martian, entirely without sentiments of compassion and piety,know about what is going on during a commemoration of the armistice?For the sake of argument, we assume complete physical predictabilityand explainability of the behavior of humans equipped with vision, asense of humor, and sentiments of piety. The Martian could thenpredict all responses, including the linguistic utterances of theearthlings in the situations which involve their visual perceptions,their laughter about jokes, or their (solemn) behavior at thecommemoration. Butex hypothesi, the Martian would be lackingcompletely in the sort ofimagery andempathy whichdepends on familiarity (direct acquaintance) with the kinds ofqualia to be imaged or empathized (1958, 431).

B. A. Farrell had earlier presented a similar thought experimentfeaturing a Martian; in this version, it is humans who lack knowledgeof what it is like for the Martian to exercise his sensory capacities(1950, 183; though Farrell ultimately argues that this thoughtexperiment does not present a challenge to physicalism). Paul E.Meehl, responding to Feigl, describes two individuals who each havecomplete neurophysiological knowledge, one of whom is congenitallyblind; he takes it as intuitive that this person does not knowsomething which the other does, namely “what red lookslike” (1966, 151).

More recent examples from the literature come closer to being versionsof the Knowledge Argument rather than merely precursors to it. Forexample, consider the following statement of the knowledge intuitionby Nicholas Maxwell:

from a complete physicalist description alone it would be impossibleto deduce the perceptual qualities of things, but this is due, not tothe fact that things do not really possess perceptual qualities, butto the fact that the physicalist description is incomplete: it doesnot tell us all that there is to know about the world. It does nottell us what it is like to be a human being alive and experiencing inthe world (1965, 309).^[2]

Howard Robinson, writing the same year as Jackson published‘Epiphenomenal Qualia’, describes a deaf scientist“who knows everything there is to know about the physicalprocesses involved in hearing”, but who intuitively does notknow what it is like to hear (1982, 4). Saul Kripke had outlinedsimilar ideas in unpublished lectures given in 1979 (Renero 2023).

Finally, it is worth mentioning the extremely influential thoughtexperiment described by Thomas Nagel (1974). According to Nagel, thephysical facts about an organism and its perceptual systems are“objective factspar excellence – the kind thatcan be observed and understood from many points of view and byindividuals with differing perceptual systems” (1974, 442).Nagel argues that even if we knew all the objective facts about abat’s sonar system, we would still not know what it would belike to perceive using this system. Therefore, complete knowledge ofthe physical facts about a bat’s perceptual system would notyield knowledge of certain facts about a bat’s experiences;these facts can only be captured from a subjective perspective. Nageldoes not argue against physicalism, but rather claims that wepresently have no understanding of how it can be true. InSection 4.9, we shall outline arelated approach to the knowledge argumentwhich interprets it as a challenge not to physicalism but to aposition called objectivism.

2. The Basic Idea

Frank Jackson (1982) formulates the intuition underlying his KnowledgeArgument in a much cited passage using his famous example of theneurophysiologist Mary:

Mary is a brilliant scientist who is, for whatever reason, forced toinvestigate the world from a black and white room via a black andwhite television monitor. She specializes in the neurophysiology ofvision and acquires, let us suppose, all the physical informationthere is to obtain about what goes on when we see ripe tomatoes, orthe sky, and use terms like ‘red’, ‘blue’, andso on. She discovers, for example, just which wavelength combinationsfrom the sky stimulate the retina, and exactly how this producesvia the central nervous system the contraction of the vocalchords and expulsion of air from the lungs that results in theuttering of the sentence ‘The sky is blue’.… Whatwill happen when Mary is released from her black and white room or isgiven a color television monitor? Will shelearn anything ornot? It seems just obvious that she will learn something about theworld and our visual experience of it. But then is it inescapable thather previous knowledge was incomplete. But she hadall thephysical information.Ergo there is more to have than that,and Physicalism is false.

The argument contained in this passage may be put like this:

(1)

Mary has all the physical information concerning human colorvision before her release.

(2)

But there is some information about human color vision that shedoes not have before her release.
Therefore:

(3)

Not all information is physical information.

Most authors who discuss the knowledge argument cite the case of Mary,but Frank Jackson used a further example in his seminal article: thecase of a person, Fred, who sees a color unknown to normal humanperceivers. We might want to know what color Fred experiences whenlooking at things that appear to him in that particular way. It seemsclear that no amount of knowledge about what happens in his brain andabout how color information is processed in his visual system willhelp us to find an answer to that question. In both cases cited byJackson, an epistemic subjectA appears to have no access toparticular items of knowledge about a subjectB:Acannot know thatB has an experience of a particular qualityQ on certain occasions. This particular item of knowledgeaboutB is inaccessible toAbecauseA never had experiences ofQ herself.

3. Some Clarifications

3.1 Two Versions of the Argument

As Horgan (1984) points out, talk of ‘physicalinformation’ in the context of the knowledge argument isambiguous between an epistemological and an ontological reading.“Physical information” may be interpreted (a) in the senseof what Horgan calls ‘explicit physical information’(according to Horgan’s proposal a sentenceS expressesexplicit physical information about certain processes just in caseS belongs to, or follows from, a theoretically adequatephysical account of those processes) or (b) in the sense of‘ontologically physical information’ which is explicatedin Horgan (1984, 150) as follows: a sentenceS“expresses ontologically physical information about certainprocesses just in case (i) all entities referred to or quantified overinS are physical entities, and (ii) all the properties andrelations expressed by the predicates inS are physicalproperties and relations.” Presupposing a distinction alongthese lines one may replace ‘to have all explicit physicalinformation aboutx’ by ‘to have completephysical knowledge aboutx’ and one may replace‘to have all ontologically physical information aboutx’ by ‘to know all the physical facts aboutx’. The argument may thus be reformulated in twodifferent ways:

(V1)

The weaker version of the knowledge argument:

(1a)

Mary has completephysical knowledge concerning facts about human color vision before her release.

(2a)

But there is somekind of knowledge concerning facts about human color vision that she does not have before her release.
Therefore:

(3a)

There is somekind of knowledge concerning facts about human color vision that is non-physical knowledge.

(V2)

The stronger version of the knowledge argument:

(1b)

Mary knows all thephysical facts concerning human color vision before her release.

(2b)

But there aresome facts about human color vision that Mary does not know before her release.
Therefore:

(3b)

There arenon-physical facts concerning human color vision.

The conclusion of the stronger version of the argument (3b) is anontological claim that the physicalist must reject. Theconclusion of the weaker version of the argument is merely anepistemological claim that is compatible with denying the existence ofnon-physical facts. Although Jackson’s original formulation interms of information is open to both interpretations it is clear thatthe second stronger version is what he had in mind.

As many have pointed out, the result of the weaker version (3a) doesnot imply the result of the stronger version (3b). That a person hasincomplete knowledge about a certain topic does not imply withoutfurther assumptions that there is some specific fact she does not haveknowledge of. The example of knowledge about oneself (de seknowledge) may illustrate the general point. Let us suppose that John,who is att in Amsterdam, does not know that he is now inAmsterdam (if asked about his present location he would assert“I am now in Venice”). John’s knowledge concerningthe present location of people is incomplete. He lacks a specificlocating piece ofde se knowledge. Still, there need not beany fact concerning the location of people that John does not haveknowledge of. It does not follow from the description of the case thatJohn does not have knowledge of the fact that John is in Amsterdam.John may well know that John is in Amsterdam but, having forgottenthat he is himself John, he may fail to conclude that he is now inAmsterdam. If John finally learns that he is in Amsterdam, he does notthereby learn a new fact – or so many philosophers would insist– he gains new knowledge of a fact that he already knew in adifferent way.

If – in analogy to thede se case – some physicalfacts about color vision can be known in two different ways, –in a ‘physical way’ (under ‘physicalconcepts’) and in some other, non-physical way (under‘non-physical concepts’), then it is possible to acquirenew (non-physical) knowledge about a (physical) fact without therebyacquiring knowledge of a new fact (the very same fact may have beenknown before under its physical conceptualization). Many authorsaccept the weaker version of the argument but reject the stronger onefor the reason just sketched: they admit that Mary gains newpropositional knowledge but deny that she thereby comes to know factsthat she did not know before in some other way. (These authors acceptthe first premise of both versions of the argument and the secondpremise of the first version as well, but they deny the second premiseof the second version and insist that (2a) does not imply (2b)). Theirposition with respect to the knowledge argument will be called theNew Knowledge/Old Fact-View (seeSection 4.6 below). Others deny even the weaker version V1 and claim that Marydoes not gain any new propositional knowledge (no new knowledge aboutsomething that is the case, no factual knowledge). Their position willbe called theNo Propositional Knowledge View (seeSections 4.3 and4.5 below).

To locate the different points of disagreement it is helpful toformulate the stronger version of the argument more explicitly.

(V3)

Explicit formulation of the knowledge argument (stronger version):

Premise P1

Mary has complete physical knowledge about human color visionbefore her release.
Therefore:

Consequence C1

Mary knows all the physical facts about human color visionbefore her release.

Premise P1

There is some (kind of) knowledge concerning facts about humancolor vision that Mary does not have before her release.
Therefore (from (P2)):

Consequence C2

There are some facts about human color vision that Mary does notknow before her release.
Therefore (from (C1) and (C2)):

Consequence C3

There are non-physical facts about human color vision.

Once C1 and C2 are accepted, there is obviously no way to avoid C3(which follows logically from the former two). Moreover, is seems hardto deny that it isin principle possible to have completephysical knowledge about human color vision (or about an appropriatelychosen part thereof). If so, premise P1 should be accepted as anappropriate description of a legitimate thought experiment. To avoidthe antimaterialist conclusion C3 the physicalist can (a) objectagainst the inference from P1 to C1 (a minority of philosophers havechosen this strategy, seeSection 4.2 below) or he or she can avoid C2 by (b) denying premise P2 (this isthe strategy chosen by proponents of theNo PropositionalKnowledge View, see Sections4.3 and4.5 below) or by (c) blocking the inference from premise P2 to C2 (thisis the strategy chosen by a majority of physicalist philosophers whosubscribe to some version of theNew Knowledge/Old Fact View,seeSection 4.6 below).

3.2 Physical and Non-physical

The knowledge argument is often cited as one of those anti-physicalistqualia-based arguments that are supposed to justify property dualism.The above formulation, however, does not explicitly mentionnon-physical properties but only non-physical facts. But the relationbetween the two claims is obvious. Friends of the knowledge argumentwill say that the facts at issue are non-physicalbecausethey involve the exemplification of non-physical properties (e.g. ofthe property of having an experience with qualityQ).

In the assumption that Mary has all physical knowledge (first version)or knows all the physical facts (second version)“physical” is meant in a very broad sense that includesknowledge about (or facts concerning) the functioning of the receptorsand neurons involved in color vision (biological and physiologicalknowledge/facts) as well as knowledge about (or facts concerning) thewhole network of causal relations between processes underlying colorvision, external stimuli and behavior (functional knowledge/functional facts). “Physical” knowledge in the broad senseat issue even includes psychological knowledge (e.g. knowledge aboutthe result of psychophysical experiments) in so far as they can beformulated without use of phenomenal terminology. One might try toexplicate “physical knowledge” in the sense at issue inroughly the following way: physical knowledge includes all knowledgethat is expressible in a terminology that does not contain irreduciblymental terms. It would be natural to define physical facts as thosefacts that can be expressed in this way. But note that this definitionof ‘physical facts’ begs the question against an objectionthat has been raised against the knowledge argument (seeSection 4.2 below).

It is certainly not easy to formulate a precise, adequate and nonquestion-begging account of “physical knowledge” and“physical facts” suited for the discussion of theknowledge argument. It is, however, quite common to assume that ourintuitive understanding of “physical knowledge” in thebroad sense at issue is clear enough for the purposes of the debate,though some argue that talk of “physical facts” needsclarification (see Alter 1998). One prominent suggestion is thatphysical knowledge is exhausted by knowledge ofstructuralinformation, “information that can be fully expressed bystatements containing only logical, mathematical, nomic, and perhapsspatiotemporal vocabulary” (Alter 2023, 11; for furtherdiscussion of structure and the closely related notion of dynamics seeChalmers 2010a; Alter 2016).

3.3 Knowing what it is like

It is common to formulate Mary’s new knowledge in terms ofThomas Nagel’s famous locution of knowing what it’s like:Mary does not know (while living in her black-and-white environment)what it is like to see colors and she learns what it is like to seecolors only after her release. But this common way to put the pointmay lead to a confusion of (a) mere acquaintance with kinds of colorexperiences by having and remembering them and (b) knowledge aboutwhat kind of color experience other subjects have at a given occasion,and it may thereby lead to a failure to distinguish two steps ofepistemic progress that Jackson’s Mary takes at once. To see thetwo steps involved one may consider an example used inNida-Rümelin (1996) and (1998): Like Mary, Marianna first (att₁) lives in a black and white environment.Contrary to Mary (at a later momentt₂) she getsacquainted with colors by seeing arbitrarily colored objects (abstractpaintings, red chairs, blue tables, etc. but no yellow bananas, nopictures of landscapes with a blue sky etc.). Marianna is thereforeunable to relate the kinds of color experiences she now is acquaintedwith to what she already knew about them att₁. Att₂, Marianna may wonder which of four slides (ared, a blue, a green and a yellow slide) appears to her in the colornormal people experience when looking at the cloudless sky. Att₂ Marianna knows, in a sense, what it is like tohave experiences of red, blue, etc. But she still lacks the relevantitems of knowledge about what other people experience: there is aclear sense in which she still may not knowthat the skyappears blue to normal perceivers, she may even have the false beliefthat it appears to normal perceivers like the red slide appears to herand thus believe, in a sense, that the sky appears red to normalperceivers. Only att₃, when Marianna is finallyreleased and sees the sky, does she gain this item of knowledge. Oneway to describe the two steps of epistemic progress is this: Att₂, by having color experiences, Marianna can formnew concepts, she now has what has been called ‘phenomenalconcepts’ of kinds of color experiences. By acquiring theseconcepts she acquires the capacity to ask new questions, and to formnew (eventually false) hypotheses (e.g. about the appearance of thesky to normal perceivers). Only att₃ does sheacquire the kind of knowledge that the knowledge argument is concernedwith (knowledge that involves the application of phenomenal concepts)about experiences of other people.

Once these two steps are clearly distinguished one may conclude thatMarianna’s relevant epistemic progress att₃(and Mary’s relevant progress after release) is not happilydescribed by talk of knowing what it’s like. Rather, or so onemay argue, Mary and Marianna acquire a particular kind of beliefthat the sky appears blue to normal perceivers, namely thephenomenal belief that it appears blue to normal perceivers, wherephenomenal belief involves the application of the appropriatephenomenal concept. Both may have believed, in a sense (thenon-phenomenal sense that does not require use of phenomenal concepts)that the sky appears blue to normal perceivers while still in theirblack-and-white environment (they may have been told so by theirfriends). (For the distinction between phenomenal and non-phenomenalbelief see Nida-Rümelin 1996 and 1998).

In addition to the danger of confusing the two steps of epistemicprogress distinguished above, talk of knowing what it is like raisesvarious semantic issues. One issue is whether attributions of knowingwhat it is like can be reduced to attributions of propositional orfactual knowledge, so that for Mary to know what it is like to F justis for her to know that p, where ‘p’ is a propositionrelated in some suitable way to F-ing (for different views on this seeLycan 1996, 92–93; Tye 2011; Lynch 2020; see also Sections 4.3–4.5. Asecond issue is raised by Daniel Stoljar, who distinguishes two waysof reading attributions of knowing what it is like (2016). On theinterrogative reading, to know what it is like to, e.g., seered just is to know some fact which answers the question ‘Whatis it like to see red?’ On thefree relative reading,to know what it is like to see red is to know a type ofexperience. Stoljar outlines a possible criticism of the KnowledgeArgument which appeals to an ambiguity between these two readings ofknowing what it is like, concluding that the Knowledge Argument can bereformulated to avoid this criticism.

4. Objections

4.1 Doubts about the Thought Experiment

Some authors have raised doubts about the thought experiment itself.It is sometimes pointed out, for example, that merely confining Maryto a monochromatic environment would not prevent her from having colorexperiences (see Thompson 1995, 264) or that, after release, she wouldnot be able to see colors. But the example can be refined to meetthese objections. Mary might be monochromatic from birth and changedinto a normal perceiver by some medical procedure. It is sometimesobjected that already accepted or future results of visual science areor might be incompatible with the existence of a Mary-case (a personwith monochromatic experience who becomes a normal color perceiverlater) or that such results might require (to preserve consistencewith visual science) the introduction of so many additionalassumptions that the conceivability of the example becomes doubtful.To this one might reply that the thought experiment need not becompatible with visual science. If the case of a person withmonochromatic vision who turns into a normal perceiver really doesinvolve serious difficulties for materialism, then the mere fact (ifit were one) that our visual apparatus excludes the actual existenceof such a case does not seem to provide a convincing reply for thematerialist. But this point (the relevance or irrelevance of visualscience in this context) has not received much discussion in theliterature. It has, however, been pointed out (see Graham and Horgan,2000, footnote 4 with its reference to Shepard 1993) that at leastpresently available results of color vision science do not exclude aMary-case. (The psychologist Knut Nordby was a real life case of acolor vision specialist who was also a complete achromat. See Nordby1996 [Other Internet Resources] and Nordby, 2007.)

Another doubt about the thought experiment is raised by the claim thata person who is confined to a monochromatic environment but knowseverything physical there is to know about visual color experiencewould be able to figure out what colored things look like and thuswould e.g. be able to imagine the kind of color experience produced innormal perceivers when looking at the cloudless sky during the day(see e.g. Dennett 1991; Dennett 2007; Churchland 1989; Maloney 1985,36). Probably the most common reaction to this is simply to doubt theclaim. But it is not clear that the claim, if correct, would underminethe knowledge argument. The opponent would have to show that completephysical knowledgenecessarily involves the capacity toimagine blue. One may doubt that this claim is compatible with thewidely accepted assumption that physical knowledge can be acquiredindependently of one’s particular perceptual apparatus.(Arguably a subject whose visual apparatus is not suited for visualexperiences at all will not be able to develop the capacity to imaginecolors on the basis of physical knowledge alone, even if this weretrue for Mary).

Some have argued that Mary would recognize the colors when firstseeing them on the basis of her complete physical knowledge aboutcolor vision (see Hardin 1992; Ross 2018). According to this claim shewould think something like “oh, so this is red” when firstconfronted with a red patch and she could not be fooled by whatDennett calls ‘the blue banana trick’: when shown a bluebanana she would know that it has the wrong color (see Dennett 1991).A possible and common response is to simply doubt these claims. But,in any case, it is not clear that these claims undermine the knowledgeargument. One may respond along the following lines: If Mary whenfirst confronted with red were able to conclude that she is now seeingwhat people call red, she therebyacquires a large set ofnew beliefs about red experiences (that they are produced byroses, such-and-such wavelength combinations and so on). On the basisof seeing red she (a) acquires a new phenomenal concept of red and (b)she forms new beliefs involving that new concept using her previouslyacquired physical knowledge. But if this description is correct, thenher previous knowledge was incomplete (for a detailed discussion ofDennett’s argument involving the blue banana trick see Dale1995).

A slightly different way of doubting the thought experiment is to ineffect shift the burden onto proponents of the Knowledge Argument. Theidea is that we are not in an informed position concerning whether ornot Mary would know the phenomenal facts, because ex hypothesi she hascomplete knowledge of the relevant physical facts and we do not; wemay be ignorant of certain physical facts which would allow Mary todeduce the relevant phenomenal facts. Thisignorancehypothesis is defended in Stoljar (2006, 79–86), primarily on thebasis of considering analogous thought experiments (for criticaldiscussion see Alter 2023, 41–50).

McClelland (2019) develops this line of thought into a dilemma. Giventhe ignorance hypothesis, the proponent of the Knowledge Argument mustoffer reasons to think that the physical facts in an ideal completedscience would not entail the phenomenal facts. If such reasons are notforthcoming, then we should not accept the Knowledge Argument. But ifthey are forthcoming then the Knowledge Argument would be redundant(since those reasons would by themselves suffice to defeatphysicalism).

It is also worth mentioning that non-philosophers seem to agree thatMary would not know everything about experiences while confined to theroom. 82% of respondents in a survey answered that Mary in the roomwould not know what it is like to see red, and this finding heldacross a range of different demographic groups (Gregory, Hendrickx& Turner 2022, 530–532).

4.2 Complete Physical Knowledge without Knowledge of all the Physical Facts

It may appear obvious that premise P1 (Mary has complete physicalknowledge about human color vision) implies C1 (Mary knows all thephysical facts about human color vision). If all physical facts can beknown under some physical conceptualization, then a person who hascomplete physical knowledge about a topic knows all the relevantphysical facts. But a few philosophers can be understood as objectingagainst precisely this apparently unproblematic step. Harman (1990)argues that Mary does not know all the functional facts concerninghuman color vision because she lacks the concept of what it is for anobject to be red, blue, etc. Flanagan (1992) distinguishesmetaphysical physicalism fromlinguisticphysicalism. While metaphysical physicalism is the ontologicalclaim that there are no non-physical individuals, properties orrelations and no non-physical facts, linguistic physicalism says that“everything physical can be expressed or captured in thelanguages of the physical sciences.” According to FlanaganMary’s case may refute linguistic physicalism but does notrefute metaphysical physicalism. Alter (1998) points out that theknowledge argument needs the premise that all physical facts can belearned discursively and argues that this assumption has not beenestablished. It may be argued against this view that it becomes hardto understand what it is for a property or a fact to be physical oncewe drop the assumption that physical properties and physical facts arejust those properties and facts that can be expressed in physicalterminology.

Moran (2023) develops a proposal like Flanagan’s which offers away to address this worry. He suggests that the physicalist candistinguish betweennarrowly physical facts, those revealedby fundamental physics, andbroadly physical facts, thosewhich are wholly and fully grounded in the narrowly physical facts.(Crucially, this distinction assumes that facts which are wholly andfully grounded in narrowly physical facts are themselves physical– for critical discussion of this assumption seeNida-Rümelin & O’Conaill forthcoming.) A physicalistwill hold that phenomenal facts about, e.g., what it is like to seered, are broadly physical. Moran argues that Mary can come to knowphenomenal facts only on leaving the room. This allows him to denythat P1 entails C1.

4.3 No Propositional Knowledge 1: the Ability Hypothesis

Two different versions of theNo Propositional Knowledge-Viewhave been proposed. According to theAbility Hypothesis (mostprominently defended in Lewis 1983, 1988 and in Nemirow 1980, 1990,2007), Mary does not acquire any new propositional knowledge afterrelease (no knowledge about something that is the case, no factualknowledge), but only a bundle of abilities (like the ability toimagine, remember and recognize colors or color experiences).According to theAcquaintance Hypothesis proposed by Conee(1994), Mary’s new knowledge after release is what he calls“acquaintance knowledge” which is neither propositionalknowledge nor identical to a bundle of abilities.

Proponents of the Ability Hypothesis presuppose that Mary’sepistemic progress after release consists in the acquisition ofknowing what it is like (e.g. to have an experience of blue) and theyclaim that to know what it is likeis to have certainpractical abilities. According to Nemirow “knowing what anexperience is like is the same as knowing how to imagine having theexperience” (1990, 495). According to Lewis,

…knowing what it is like is the possession of abilities:abilities to recognize, abilities to imagine, abilities to predictone’s behavior by imaginative experiments (Lewis 1983, 131).

A few years later he writes:

The Ability Hypothesis says that knowing what an experience is likejustis the possession of these abilities to remember,imagine, and recognize. … It isn’t knowing-that.It’s knowing-how (Lewis 1990, 516).

Bence Nanay suggests that what Mary acquires is the ability todiscriminate between different types of awareness, i.e., todistinguish having or imagining experiences of typeE fromhaving or imagining experiences of other types (2009).

Lewis’s main argument for the Ability Hypothesis can besummarized like this. (1) The only alternative to the AbilityHypothesis is what he calls theHypothesis of PhenomenalInformation (HPI). (According to the HPI knowing what it is likeis propositional in the following sense: coming to know what it islike involves the elimination of hitherto open possibilities). (2) TheHPI is incompatible with physicalism. (3) The Ability Hypothesis iscompatible with physicalism and explains everything that may beexplained by the HPI. Therefore: The Ability Hypothesis should bepreferred.

Note that the Ability Hypothesis is compatible with the view that wedo sometimes acquire propositional knowledge on the basis of gettingacquainted with a new kind of experience from the first personperspective. The following remarks by Levin are hard to deny:

…it would be perverse to claim that bare experience can provideus only with practical abilities…. By being shown an unfamiliarcolor, I acquire information about its similarities andcompatibilities with other colors, and its effects on other mentalstates: surely I seem to be acquiring certain facts about color andthe visual experience of it (Levin 1986, 246; see also Crane 2003).

But, as pointed out by Tye (2000), this does not undermine the AbilityHypothesis. The Ability Hypothesis implies that there is someknowledge that canonly be acquired by having experiences ofa particular kind and thatthis knowledge is nothing butknowing-how. This of course does not exclude that therealsoispropositional knowledge thatcan be acquired bygetting acquainted with kinds of experiences from the first personperspective. The proponent of the Ability Hypothesis only has toinsist that,if there is such propositional knowledge, thenitneed not be acquired on that particular basis but isaccessible in other ways as well.

4.4 Objections Against the Ability Hypothesis

It has been argued against Nemirow that the ability to imagine havingan experience of a particular kind is neither necessary nor sufficientfor knowing what it is like to have that kind of experience. To showthat imaginative abilities are not necessary for knowing what it islike, Conee (1994) and Alter (1998) cite the example of a person whohas no capacity to imagine having color experiences. They claim thatdespite this defect she would know what it is like to have anexperience of e.g. green while attentively staring at something thatlooks green to her. To show that imaginative abilities are notsufficient for knowing what it is like Conee introduces the followingexample: A person, Martha, “who is highly skilled at visualizingan intermediate shade that she has not experienced between pairs ofshades that she has experienced…happens not to have anyfamiliarity with the shade known as cherry red.” Martha is toldthat cherry red is midway between burgundy red and fire red (she hasexperienced the latter two shades of red). Given this information andher extraordinary capacity, Martha has the ability to imagine cherryred, but as long as she does notexercise this ability shedoes not know what it is like to see cherry red.

A similar example is used for the same purpose and discussed in moredetail by Raymont 1999. Raymont argues that mnemic, recognitional andimaginative abilities neither separately nor conjointly amount toknowing of what it is like to have a particular kind of experience. Hefirst argues that none of these abilities is necessary and sufficientfor knowing what it is like: (a) Mnemic abilities are not necessary,since someone can learn what an experience is like when first havingit without already remembering an experience of the relevant kind. (b)Imaginative abilities are not sufficient since someone can have theability to imagine a particular kind of experience without exercisingit (see the example cited above). (c) To show that recognitionalabilities are not sufficient either, Raymont cites empirical data“in support of the view that one can have the ability tononinferentially recognize a certain type of visual experience withoutever having had it, and thus without knowing what it is like to haveit”. But then these three kinds of abilities cannot conjointlyamount to knowing what it is like either: if they did, then –contrary to (a) – each of them would have to be a necessarycondition for knowing what it is like.

A different argument against appeals to imagination is offered by AmyKind (2019). She argues that Mary could while in the room alreadyimagine seeing red, so we cannot explain her learning anything whenshe leaves the room by appealing to her acquiring a new ability toimagine. Kind also discusses reformulations of the Ability Hypothesis,e.g., in terms of Mary acquiring the ability to imagine correctly, andargues that these also face objections (for discussion seeO’Conaill 2020).

Gertler (1999) argues that the best candidate for an analysis in thespirit of the Ability Hypothesis is to identify knowing what it islike to have an experience of red with the ability to recognizeseeing-red experiences by their phenomenal quality and then goes on toattack this candidate: she points out that the ability to recognizeseeing-red experiences by their phenomenal quality can be explained bythe fact that I know what it is like to see red but notvice versa.^[3]

Michael Tye (2000) concedes that none of the abilities considered byLewis is necessary for knowing what it is like and he discusses thefollowing possible revision of the Ability Hypothesis: knowing what itis like to have an experience of red is the ability to apply anindexical concept to an experience of red (while having it) viaintrospection. But, he goes on to argue, this revised version canagain be rejected by a counterexample that shows that the ability atissue is not sufficient for knowing what it is like: If Mary isdistracted and does not attend to her experience when she first sees ared object, then she need not apply any concept to her experience atall. In this case, she still does not know what it is like to have redexperiences although she has theability to apply anindexical concept to her present experience (she has the ability, but,being distracted, she does not exercise it). Tye concedes that therevised version of the Ability Hypothesis could not, anyway, be usedagainst the knowledge argument in the way that was originallyintended. The reason is that the revised version is compatible withthe view that Mary does acquire knowing-that if she is not distractedwhen first seeing something red: she learns thatthis is ared experience (where “this” refers introspectively to herpresent experience) and so acquires knowing-that. According to Tye tohave indexical knowledge of this kind is sufficient but not necessaryfor knowing what it is like to have a red experience. After all, it isimpossible to introspectively refer to a red experience withoutpresently having that kind of experience, but Tye wishes to concedethat a person can know what it is like to have a red experience whilenot presently having a red experience. This reasoning motivates hisdisjunctive account of knowing what it is like: “Sknows what it is like to undergo experienceE iff eitherS now has indexical knowledge-that with respect toEobtained via current introspection orS has the Lewisabilities with respect toE” (Tye 2000). Tye thusdefends the physicalist view against the knowledge argument by acombination of the two strategies mentioned above: he applies theNew Knowledge/Old Fact-strategy to the person who knows whatit is like to have an experience in the sense of the first conjunct(the indexical thought at issue is made true by a physical fact) andhe applies theNo Propositional Knowledge-strategy to thecase of someone who has knowing what it is like in the sense of thesecond disjunct.

Lycan (1996) argues against the Ability Hypothesis and for the viewthat Mary acquires new knowledge-that after release by claiming that“S knows what it is like to see blue” meanssomething like “S knows that it is likeQ tosee blue” whereQ names the phenomenal quality atissue. It has been objected by Tye (1995) that the use of the qualianame “Q” within a propositional attitude contextcreates the well-known problems: Replacing “Q” byanother name “R” for the same quale may changethe truth value of the belief ascription. A proponent of Lycan’sview could however respond along the following lines: In the case ofqualia names within belief contexts it does not matter which name isused to refer to the quale at issue as long as the belief is meant inthe sense of a phenomenal belief ascription. “Sbelieves that it is likeQ to see blue” means, on thephenomenal reading, thatS has the relevant belief aboutQ under a phenomenal concept ofQ. Under theassumption that it is impossible to have two different phenomenalconcepts of one and the same quale, the objection is met: As long astwo qualia namesQ andR refer to the same quale,replacingQ byR in an ascription of phenomenalbelief cannot change the truth value of the belief ascription.

As we have seen, proponents of the Ability Hypothesis assume that theknow-how which Mary acquires is distinct from any propositionalknowledge. This assumption can be challenged, based for example on thework of Jason Stanley and Timothy Williamson (2001). On this view, fora subjectS to know how to do something (to F) is forS to know that there is a wayw forS to F,and forS to know this under a practical mode of presentation(2001, 430). Stanley & Williamson themselves apply this thought toLewis’s version of the Ability Hypothesis:

Knowing how to imagine red and knowing how to recognize red are bothexamples of knowledge-that. For example,x’s knowinghow to imagine red amounts to knowing a proposition of the form‘w is a way forx to imagine red’,entertained under a guise involving a practical mode of presentationof a way (2001, 442; see also McConnell 1994).

Yuri Cath suggests that this point can be accommodated by proponentsof the Ability Hypothesis, provided they distinguish betweenMary’s learning a new proposition and her coming to be in newstate of propositional knowledge (2009, 142–143). Specifically,before her release Mary can know thatw is a way for someoneto imagine red, but only under a theoretical mode of presentation;after her release, she comes to know the same proposition under apractical mode of presentation. She thus comes to be in a new state ofpropositional knowledge, but without learning any new propositions.^[4]

4.5 No Propositional Knowledge 2: The Acquaintance Hypothesis

Earl Conee (1994) proposes another variant of theNo PropositionalKnowledge-View. According to Conee acquaintance constitutes athird category of knowledge that is neither reducible to factualknowledge nor to knowing-how and he argues that Mary acquires afterrelease only acquaintance knowledge. According to Conee knowingsomething by acquaintance “requires the person to be familiarwith the known entity in the most direct way that it is possible for aperson to be aware of that thing” (1994, 144). Since“experiencing a quality is the most direct way to apprehend aquality” (Conee 1994, 144), Mary gains acquaintance with colorqualia only after release. According to the view proposed by Conee thephysicalist can defend himself against the knowledge argument in thefollowing way: (1) Qualia are physical properties of experiences (andexperiences are physical processes). LetQ be such aproperty. (2) Mary can know all aboutQ and she can know thata given experience hasQ before release, although –before release – she is not acquainted withQ. (3)After release Mary gets acquainted withQ, but she does notacquire any new item of propositional knowledge by getting acquaintedwithQ (in particular she already knew under what conditionsnormal perceivers have experiences with the propertyQ). Morerecently Michael Tye (2009, 131–137) defends the acquaintancehypothesis as the right answer to the knowledge argument therebyabandoning his original response (see below 4.7).

A friend of the knowledge argument might concede that a person isacquainted withQ only if she has or had anexperience with propertyQ but she could insistthat being acquainted withQ in that sense enablesMary to acquire a new item of propositional knowledge, e.g., knowing(in the relevant sense) that an experience hasQ (see,e.g., Alter 2023, 80–83). Another kind of criticism of theAcquaintance Hypothesis is developed in Gertler (1999). She arguesthat the property dualist can explain why the most direct way to getfamiliar with a quale is by having an experience of the relevant kindwhile the physicalist does not have any explanation for thisparticular feature of qualia. Similarly, Robert Howell (2007, 146)argues that all other physicalist responses to the Knowledge Argumentdepend upon the Acquaintance Hypothesis, and that acquaintance isincompatible with objectivism (seeSection 4.9 below).

It is interesting to see that one version of the New Knowledge/OldFact View is very similar in spirit to the Acquaintance Hypothesis.Bigelow and Pargetter (1990) and Grzankowski and Tye (2019) argue thatMary’s progress after release consists in the fact that she nowstands in a new acquaintance relation to color qualia, but theirtheory about the individuation of beliefs implies that she therebyacquires new factual knowledge. On this view, different beliefs can bedistinguished appropriately only if one takes into consideration theway the subject is acquainted with the individuals and properties herbelief is about (for instance, Bigelow and Pargetter distinguishbetween different “modes of acquaintance” in thatcontext).^[5]

4.6 The New Knowledge/Old Fact View

Several positive arguments for the view that Mary’s newknowledge after release constitutes propositional knowledge (genuineinformation) have been formulated in the literature. Lycan argues, forexample, that Mary’s new knowledge goes along with theelimination of epistemic possibilities and that her new abilities arebest explained by her having new information (for further argumentssee Lycan 1996, 92). Loar (1990/1997) points out that the embeddedoccurrence of “feels like such and such” in sentences like“if pains feel like such and such thenQ” cannot be accounted for in a model that treatsknowing what it is like as mere know-how. McConnell (1994) defends themore radical view that the acquisition of knowing-how is normallyaccompanied by the acquisition of a particular new item ofknowing-that.

Many philosophers find it hard to deny that Mary gains new factualknowledge after release and for that reason (if they are physicalists)feel attracted by the New Knowledge/Old Fact View. Positions thatclearly fall into that category are defended in Horgan 1984;Churchland 1985; Tye 1986, 1995; Bigelow and Pargetter 1990; Loar1990/1997; Lycan 1990a, 1996; Pereboom 1994; Perry 2001; Byrne 2002;Papineau 2002, 2007; Van Gulick 2004; Levin 2007; Balog 2012a,2012b.

The basic ideas common to the New Knowledge/Old Fact View may besummarized as follows:

(1)

Phenomenal character, e.g. phenomenal blueness, is aphysicalproperty of experiences (but see Lycan 1990a for an exception whoconstrues qualia as properties of external objects).

(2)

To gain knowledge of what it is like to have an experience of aparticular phenomenal character requires the acquisition ofphenomenal concepts of phenomenal character.^[6]

(3)

What it is for an organism to acquire and possess a phenomenalconcept can be fully described in broadly physical terms.

(4)

A subject can acquire and possess phenomenal concepts only if ithas or has had experiences of the relevant phenomenal kind.

(5)

After release Mary gains knowledge about phenomenal charactersunder phenomenal concepts.

But the facts that make these new items of knowledge true are physicalfacts that Mary knew before release under anotherconceptualization.

The differences between variants of the New Knowledge/Old Fact Viewconcern the theoretical (physicalist) account of (a) phenomenalcharacter, (b) phenomenal concepts of phenomenal characters and (c)the relation between phenomenal characters and the correspondingphenomenal concepts. All proponents of the view point out that,according to their proposal, physical concepts and phenomenal conceptsare cognitively independent: it is impossible to seea priorithat something that falls under aphysical concept of aparticular phenomenal character also falls under the correspondingphenomenal concept of that phenomenal character. This is whyit is possible to have (like Mary) complete physical knowledge aboute.g. phenomenal blueness (you know everything there is to know aboutphenomenal blueness under its physical conceptualization) withouthaving a phenomenal concept of blueness and without knowing any ofthese facts under a phenomenal concept of blueness. Some have arguedthat the phenomenal conceptualization is not expressible in language(see Byrne 2002 and Hellie 2004).

In general, if a philosopherA claims that the argument ofphilosopherB does not go through, it is a point in favor ofhis view if he can provide an error theory, that is if he can explainwhy the argument mayappear correct in the first place. TheNew Knowledge/Old Fact View can claim to have an error theory withrespect to the knowledge argument. Given the cognitive independence ofphysical and phenomenal concepts of blueness itappears as ifwe could imagine a situation where everything Mary knew before releasewere fulfilled but not what she came to know after release (and thiscan be taken to imply that shedoes come to know new facts).But, according to the New Knowledge/Old Fact View this is an illusion.There is no such possible situation. What Mary learns after release ismade true by a physical fact that she already knew before her release.Some versions of the New Knowledge/Old Fact-View will be brieflydescribed in what follows.

4.7 Variants of the New Knowledge/Old Fact View

Horgan (1984) does not provide a developed theoretical account ofphenomenal concepts but is one of the first to formulate the basicintuition shared by most or all proponents of the New Knowledge/OldFact View: By having experiences of blue, Mary gets acquainted withphenomenal blueness (which is in fact a physical property ofexperiences) “from the experiential perspective,” shegains what he calls “the first person ostensive perspective onthat property” (Horgan 1984, 151): she now can refer tophenomenal blueness by thinking or saying “that kind ofproperty” while having, remembering or imagining a blueexperience and while attending to its particular quality. She thus hasacquired a newconcept of phenomenal blueness. Using this newconcept she can form new beliefs (and acquire new knowledge) aboutphenomenal blueness. Formulated in this way, the view may appearsimilar to Conee’s acquaintance account. According to bothviews, Mary’s progress consists primarily in getting acquaintedwith phenomenal blueness from an inner perspective. But contrary toConee’s thesis, according to the New Knowledge/Old Fact View,acquaintance with phenomenal blueness from an experiential perspectiveenables the subject to form anew concept of phenomenalblueness and thereby implies the capacity to acquirenewbeliefs.

An example of a more explicit theoretical account of phenomenalcharacter, phenomenal content and their relation can be found in Tye(1995). He proposes a representationalist account of phenomenalcharacter. For a state to have phenomenal character is to representinternal or external physical items in an ‘abstract’ andnonconceptual way that is “appropriately poised for use by thecognitive system” (see Tye 1995, 137–144). According toTye, there are two kinds of phenomenal concepts: indexical concepts(an example is the concept applied when thinking of a particular shadeof red as “this particular hue” while having a redexperience) and what he calls “predicative phenomenalconcepts” that are based on the capacity to make certaindiscriminations. Tye wishes to accommodate the natural intuition thatMary before release cannot fully understand the nature of phenomenalblueness (she doesn’t really know what it is to have a blueexperience). One might think that his view is incompatible with theintuition at issue. Phenomenal blueness, according to his view, has aphysical nature and one might expect that physical natures are fullydescribable in physical terms and fully understandable under aphysical conceptualization. But Tye has a surprising response:although phenomenal blueness has a physical nature, a person cannotfully understand its nature unless she thinks of phenomenal bluenessunder a phenomenal concept.

Another representationalist view about phenomenal character iscombined with the New Knowledge/Old Fact View in Lycan (1990a) and(1996). Lycan’s account of Mary’s epistemic progress canbe put, roughly, like this: Only after release Mary can form“introspective second order representations” of her owncolor experiences. One may think of an introspective representation asof “a token in one of the subject’s languages of thought,his or her Introspectorese”. It has often been said that whatMary learns is in some sense “ineffable”, that it cannotbe communicated in public language. Lycan is led to a similarconclusion within his computational theory. In his view, when Maryfinally has an experience of blue she “tokens a semanticallyprimitive mental word for the type of first-order state being inwardlysensed”, where this word in Mary’s language of thought hasan “inferential and/or conceptual role” that is“unique to its subject, in that no other subject could deploy afunctionally similar representation whose designatum was that (thesubject’s) very same first-order state-token, ….”And he concludes that “the introspective word would certainlynot be synonymous with any primitive or composite expression of publicEnglish,…” (Lycan 1996, 101).

Papineau (1996) distinguishes third person and first person thoughtsabout experiences. First person thoughts involve the imagination of anexperience of the relevant kind. The basic idea may be put like this:When Mary is finally released and after some time sufficientlyacquainted with color experiences she can ‘reproduce’ blueexperiences in her imagination. These imaginations of experiences of aparticular kind can be used to refer to experiences of the kind atissue and to think about them. Obviously, Mary could not have firstperson thoughts about color experiences (she could not use imaginedblue experiences in order to refer and to think about blueexperiences) before she ever had blue experiences. After release, Marycan acquire new beliefs: first person beliefs about blue experiences.But for every such newfirst person belief about a given kindof experience, there will be one of her oldthird personbeliefs which refers to the same kind of experience and has the samefactual content.

Another way to understand phenomenal concepts is as a species ofindexical concepts. For instance, Perry (2001) argues thatMary’s new knowledge after release does not pose a problem tophysicalism any more than indexical thoughts like “I am aphilosopher” or “today is Sunday” (for a defense ofthis claim see also McMullen 1985). Perry treats Mary’s newknowledge as a particular case of demonstrative belief (and heproposes an account of Mary’s new beliefs after release in termsof his theory of token-reflexive thoughts). After release, when seeingthe sky, Mary may think “Oh, so having blue experiences is likethis” where “this” refers to a physicalproperty (the phenomenal character) of her present color experience.She could not have had a demonstrative belief of this kind beforerelease. But, again, the fact that makes the thought true is simplythe fact that blue experiences have the particular physical propertyat issue. Therefore, she does not learn any new fact.

Doubts about Perry’s proposal have been raised along thefollowing lines. In normal cases of demonstrative reference thedemonstrated object is in some way given to the epistemic subject(when pointing to a table and referring to it by “thistable”, the object may be given as “the next table left tome”). But what is the way the kind of experience is given toMary when she thinks of phenomenal blueness under the demonstrativeconcept “this kind of experience?” It cannot be the way itfeels to have an experience with that property, since this solution,so one may argue, introduces phenomenal characters of phenomenalcharacters and thus reintroduces the original problem. Maybe“the kind of experience I am now having” is theappropriate candidate. But there are problems with this proposal too(see Chalmers 2002). There are also problems with suggesting that thedemonstrative concept is ‘thin’, in the sense that it hasno reference-fixing mode of presentation associated with it (for someof these problems see Demircioglu 2013, 263–269).

Robert Stalnaker (2008) raises further doubts regarding Perry’streatment of Mary’s new knowledge. Drawing on a distinctionbetween the two stages of Mary’s epistemic progress (seeSection 3.3 above), he argues that thecases of acquiring demonstrative beliefs which Perry compares withMary’s new knowledge are akin to the second stage ofMary’s progress, but “It is at stage one that theproblematic cognitive achievement – the learning ‘what itis like’ to see red – takes place” (2008, 44).

Stalnaker suggests an alternative way in which Mary’s newknowledge is demonstrative in character. On this proposal, what Maryacquires is essentially contextual information, where “thecontent of what is expressed or believed in a context is notdetachable from the context in which it is expressed orbelieved” (2008, 81). As an example of such information,Stalnaker describes a bomb disposal expert pointing at the ground andsaying “A bomb is buried there, and unless we defuse it now itwill explode within five minutes” (2008, 85). This information,Stalnaker suggests, cannot be known by someone who was not in thatsituation at that time, even if they know the exact co-ordinates ofwhere the bomb is located and when it will detonate. Daniel Stoljar(2011) argues that phenomenal knowledge is not essentially contextual.In defence of this claim, he suggests there is an importantdis-analogy between the difference between Mary, pre- andpost-release, and the difference between the bomb disposal expert andanyone who was not with her when she made her utterance. The thoughtwhich the bomb disposal expert expressed in her utterance could noteven be entertained by someone who was not present at that time; it isfor this reason that someone not there could not know what the expertknew. In contrast, Stoljar suggests that “the fact thatpost-release Mary learnt could have been entertained by pre-releaseMary (e.g., she can wonder whether it obtains)” (2011, 441).This suggests that what Mary learns upon release is not essentiallycontextual, at least not in the sense which Stalnaker has in mind.

Another worry about demonstrative accounts is that they do not seem todo duty to the way in which the subjective character itself is presentto the mind of the thinker when employing a phenomenal concept of thatcharacter. This worry is sometimes put in terms of acquaintance: thespecific way in which the thinker is acquainted with the referent ofher thought in using phenomenal concepts does not seem to be capturedby the demonstrative account (see Levine 2007; Howell 2007,164–166). Several attempts have been made to answer objectionsof this kind. Papineau (2002) and Balog (2012a) argue that thecognitive intimacy to be accounted for is well explained by aquotational theory of phenomenal concepts: in thoughts involvingphenomenal concepts token experiences are used in order to refer tothe kind those tokens belong to. Levine (2007) argues that even theserefined theories do not account for the specific intimate way in whichthe thinker is related to the referents of phenomenal concepts.Contrary to this Levin (2007) does not see any need to‘embellish’ the original simple demonstrative account.While Balog’s current position is an elaborated version of thequotational account, Papineau has abandoned the quotational theory andargues in Papineau (2007) that phenomenal concepts are special casesof perceptual concepts where perceptual concepts do not involvedemonstration.

An influential view about phenomenal concepts which answers theknowledge argument admitting that Mary gains new knowledge but noknowledge of new facts is developed in Loar (1990/1997): Phenomenalconcepts are recognitional concepts. To have the phenomenal concept ofblueness is to be able to recognize experiences of blueness whilehaving them. The recognitional concept of blueness refersdirectly to its referent (the physical property of blueness)where this means (in Loar’s terminology): there is no otherproperty (no property of that property) involved in the referencefixing. According to Loar’s view the recognitional concept ofphenomenal blueness refers to the physical property phenomenalblueness in virtue of being ‘triggered’ by that property.It has been doubted that ‘directness’ in Loar’ssense provides an account for what one might call acquaintance: forthe way in which the phenomenal character is present to the mind whena thinker employs phenomenal concepts (see Levine 2007). White (2007)argues against Loar that the account cannot explain the a posterioricharacter of mind-brain identity statements in a satisfyingmanner.

4.8. Objections Against the New Knowledge/Old Fact View

An objection to the New Knowledge/Old Fact View can be made asfollows. In standard cases, if a subject does not know a given fact inone way that it does know in some other way, this can be explained bytwo modes of presentation: the subject knows the fact under one modeof presentation and does not know it under some other mode ofpresentation. So, for example, a person may know the fact that Venusis a planet under the mode of presentation associated with “themorning star is a planet” and fail to know the very same factunder the mode of presentation associated with “the evening staris a planet.” In this particular case, as in many others, thedifference in the mode of presentation involves two differentproperties that are used to fix the referent. In one mode ofpresentation Venus is given as the heavenly body visible late in themorning (or some similar property), whereas in the other mode ofpresentation the object is given as the heavenly body visible early inthe evening.

If the New Knowledge/Old Fact View involves two modes of presentationof this sort, then it cannot be used to defend physicalism becausethis kind of explanation of the supposed double epistemic access tofacts concerning phenomenal types would reintroduce non-physicalproperties at a higher level: the subject would have to be describedas referring to the phenomenal type at issue by some physical propertyin case it believes the relevant fact under its physical mode ofpresentation and as referring to that phenomenal type by somenon-physical property in case it believes the relevant fact under itsphenomenal mode of presentation.

It has been argued by several authors that the different modes ofpresentation at issue in the case of beliefs about phenomenal statesdo involve the introduction of different reference-fixing propertiesand that therefore the proposal is unsuccessful. Arguments of thatkind are found in Lockwood (1989, chap. 8) and McConnell (1994). White(2007) develops the objection in detail. Block (2007) gives a detailedanswer to White (2007) based on a distinction between what he labelscognitive and metaphysical modes of presentation. Chalmers (1996,2002, 2010b) makes a similar point as White (2007) using his frameworkof primary and secondary intensions. In that framework, primaryintensions describe the way a concept picks out its referent in theactual world and the cognitive independence of phenomenal and physicalconcepts is explained by their different primary intensions. If onesingular fact can be known under a physical mode of presentation aswell as under a phenomenal mode of presentation, then the two items ofknowledge involve two concepts (a phenomenal and a physical concept)with different primary intensions and these different primaryintensions correspond to different properties.

A two-dimensional framework is used in a different manner inNida-Rümelin (2007) to develop the idea that the nature ofphenomenal properties is present to the mind of the thinker when usingphenomenal properties – an idea which leads to the result thatthe New Knowledge/Old Fact View is mistaken. This idea is alsosuggested by Philip Goff (2017). He suggests that the knowledgeargument does not by itself refute physicalism because it does notovercome the New Knowledge/Old Fact View. However, things aredifferent if the phenomenal concept which Mary acquires istransparent (i.e., it reveals the nature or essence of thephenomenal property which satisfies it):

in this case Mary’s new knowledge is knowledge of the nature ofred experiences, but if pure physicalism is true, she already knew thecomplete nature of red experiences in knowing the pure physicaltruths, and hence there ought to be nothing more she can learn abouttheir nature (2017, 74–75; see also Fürst 2011,69–70; Demircioglu 2013, 274–275).

A general argument against the materialist strategy of appealing tophenomenal concepts is developed in Chalmers (2004; 2007); forcritical discussion see Balog 2012b; Alter 2023, 144–148.

Anyone who wishes to argue in the way just mentioned, that the twomodes of presentation do involve the introduction of two differentreference-fixing properties, must deal with Loar’s proposal (see4.7). Loar avoids the problem of two reference fixing properties byhis claim that phenomenal concepts refer directly to their referent.It has been argued against Loar that his causal account of howphenomenal concepts manage to directly refer to their referent (namelyby being triggered by them) cannot appropriately describe theparticular cognitive role of phenomenal concepts (see McConnell 1994and White 2007).

A final issue concerning both the New Knowledge/Old Facts View and theknowledge argument itself is whether there are any phenomenalconcepts. Derek Ball (2009) and Michael Tye (2009) argue that thereare no such concepts, at least as defined above: in particular, bothdeny claim (4) outlined inSection 4.6, that a subject canpossess phenomenal concepts only if it has or has had experiences ofthe relevant kind. Ball and Tye appeal to work on social externalismregarding the content of our concepts to argue that even prior to herrelease, Mary possesses the same concepts which she uses to thinkabout her experiences after her release. Specifically, she possessessuch concepts ‘deferentially’, in virtue of interactingwith her linguistic community; a crucial part of social externalismabout content is that one can possess concepts deferentially eventhough one is largely ignorant of the nature of what satisfies theseconcepts (Ball 2009, 947–954; Tye 2009, 63–70).^[7] This result threatens to undermine the New Knowledge / Old FactsView, but Ball suggests it also undermines the knowledge argumentitself, since the knowledge argument itself depends on there beingphenomenal concepts (2009, 940–943).

Torin Alter (2013) responds to these arguments by focusing on thedistinction between deferential and non-deferential concept possession(the latter isconceptual mastery). He suggests that it ismastery of phenomenal concepts rather than mere deferential possessionwhich matters for the knowledge argument: “Mary makes epistemicprogress when she leaves the room because she comes to master ornon-deferentially possess phenomenal colour concepts” (2013,486). And defenders of the knowledge argument can claim that masteryof a phenomenal concept requires actually having experiences with therelevant phenomenal character. For further discussion ofconceptual mastery see Ball 2013; Alter 2019; Rabin 2019; fordifferent responses to Ball and Tye’s arguments see Veillet2012; Byrne 2021.

4.9 The Knowledge Argument and Objectivism

The Knowledge Argument has traditionally been understood as anargument against physicalism or perhaps against reductive versions ofphysicalism. But an influential alternative approach sees the argumentas working not against physicalism per se, but against a differentposition which can be termedobjectivism.^[8] Objectivism is the view that an objective description of what existscan be complete; that there are no aspects of reality which can onlybe understood by having experiences of a specific type. This notion isclosely related to Nagel’s characterization of the physicalnature of organisms as “a domain of objective factsparexcellence – the kind that can be observed and understoodfrom many points of view and by individuals with differing perceptualsystems” (1974, 442). On this view, the point of Jackson’sthought experiment is to bring out that Mary learns something whichcan only be known by having an experience of a certain kind (e.g., aperceptual experience). If this is correct, then this shows that noobjective description of what exist can be complete. As Howellformulates it, the argument understood in this way runs asfollows:

before leaving the room, Mary knew all the objective information aboutthe world. When she left the room, she gained further understandingabout the world. Therefore, all the objective information about theworld is insufficient for a complete understanding of the world (2007,147).

A number of proponents of this interpretation of the KnowledgeArgument suggest that it is compatible with a specific form ofphysicalism, on which all facts are physical or metaphysicallynecessitated by microphysical facts but some facts can only beunderstood by having specific experiences (see, e.g., Kallestrup 2006;Howell 2007; for slightly different formulations see Crane 2003; Zhao2012). That said, it is worth asking how the ‘physical’ isto be understood, such that facts which can only be known if one hascertain experiences can count as physical (see also 4.2). Coleman(2022) addresses this problem by distinguishing between two senses of‘objective’. In the first sense, an objectiveunderstanding of a property requires that it can in principle beunderstood by any subject with sufficient cognitive and sensoryapparatus (this is very similar to Nagel’s characterisation ofobjective facts cited above). In the second sense, an objectiveunderstanding of a property requires that it can in principle beunderstood without the need for having any specific kind ofexperience. Coleman argues (appealing to Jackson’s other thoughtexperiment, that of Fred – seeSection 2 that these twosenses of ‘objective’ can come apart, and that the firstsense is all that is required to connect physicalism with objectivismin a way which blocks the Knowledge Argument.

It is also worth questioning whether a view on which facts are eitherphysical or metaphysically necessitated by microphysical facts therebycounts as a form of physicalism. It has been argued that such a viewis compatible with non-physicalist positions such as emergentism orcertain forms of ethical non-naturalism (see Horgan 1993, 559-566;2010, 311–314; Crane 2010; for discussion see Stoljar 2017, section9).

5. The Dualist View About the Knowledge Argument

There has not been much discussion of the knowledge argument from adualist perspective. This is unsurprising given the small number ofcontemporary philosophers who defend a dualist position (for aprominent exception see Chalmers (1996); the knowledge argument isdiscussed on pp. 140–146). There are two possible strategies fora dualist to take who wishes to defend the knowledge argument. Thefirst is merely defensive or ‘destructive’ in that ittries to refute the positive theoretical proposals one by one thathave been used by physicalists in their objections against theknowledge argument. The second is more ‘constructive’ inthat it aims at developing an alternative positive dualist account ofphenomenal concepts, phenomenal properties and their relations suchthat on that account Mary does learn new and nonphysical facts uponrelease. Examples (or partial examples) for the first strategy mayoccasionally be found in the literature (compare Warner 1986, Gertler1999, Raymont 1995, 1999 and Connell 1994). Examples for the secondare hard to find, but Chalmers (1996, 2002) and Nida-Rümelin(2007) exemplify the second strategy. Using his framework of primaryand secondary intensions he develops a positive account of what hecalls “pure phenomenal concepts” that can be described asincorporating the old and natural intuition that in the case of qualia(phenomenal characters) there is no distinction between appearance andreality, in other words: qualia ‘reveal their nature’ inexperience.

The intuitive idea just mentioned has been expressed in differentways. Some say that qualia ‘have no hidden sides’. Otherssay that qualia are not natural kind terms in that it isnotup to the sciences to tell us what having an experience of aparticular kind amounts to (we know what it amounts to by having themand attending to the quality at issue). It is quite clear that anaccount of this intuitive idea has to be one of the ingredients of adualist defense of the knowledge argument. Nida-Rümelin (2007)develops a technical notion of grasping properties which is intendedto serve the purposes of dualists who argue against materialism usingthe assumption that in the special case of phenomenal concepts therelation that the thinker bears to the property he conceptualizes ismore intimate than in other cases: the thinker understands what havingthe property essentially consists in. This idea can be used to blockfamiliar objections to the knowledge argument in particular thosefalling into the Old fact/ New Knowledge category. A similar basicidea but formulated within a different theoretical framework iselaborated in Stephen White (2007).

According to mainstream opinion the most serious problem for propertydualism is the danger of being driven into epiphenomenalism. Ifphenomenal characters are non-physical properties and if everyphysical event has a physical cause and if we exclude the possibilityof overdetermination (where something is caused by two differentcauses that are both sufficient), then, arguably, whether or not astate has a particular phenomenal character cannot have any causalrelevance. But if qualia are causally impotent, how can a person knowthat she has an experience with a particular phenomenal character?Many take it to be obvious that a person cannot know that she now hasa blue experience unless her blue experience plays a prominent causalrole in the formation of her belief at issue. This particular problemhas been formulated as an objection against the knowledge argument inWatkins (1989). Until some time ago Jackson was one of the very fewphilosophers who embraced epiphenomenalism. But Jackson changed hismind. Jackson (1995) argues that knowledge about qualia is impossibleif qualia are epiphenomenal and he concludes that something must bewrong with the knowledge argument. In Jackson (2003) and Jackson(2007) he argues that the argument goes wrong in presupposing a falseview about sensory experience and that it can be answered by endorsingstrong representationalism: the view that to be in a phenomenal stateis to represent objective properties where the properties representedas well as the representing itself can be given a physicalist account.Jackson admits that there is a specific phenomenal way of representingbut he now insists that the phenomenal way of representing can beaccounted for in physicalist terms. Doubts about the latter claim aredeveloped in Alter (2007). Other possible reactions to the threat ofepiphenomenalism for dualism would be either to doubt that a propertydualist must embrace epiphenomenalism or to develop an account ofknowledge about one’s own phenomenal states that does not implya causal relation between qualia and phenomenal knowledge about qualia(see Chalmers 2002).

6. Further Arguments

In addition to stimulating a wide range of responses, the KnowledgeArgument has also served as an inspiration for a number of otherarguments. Mørch (2019) offers a parallel argument againstphysicalism, theexplanatory knowledge argument. She arguesthat phenomenal knowledge provides explanations of certainregularities (e.g., people will tend to avoid pain because of how itfeels) which differ from any explanations which can be provided byphysical knowledge. She argues that this difference between phenomenaland physical knowledge supports the view that phenomenal facts aredistinct from physical facts.

Robert Howell (2019; 2023, 9–15) describes a thought experiment whichresembles Jackson’s in a number of respects, in which a persontasked with discovering whether robots are capable of enjoyingconscious experiences discovers (spoiler alert) that he himself is arobot. Howell teases out the various analogies and disanalogiesbetween Jackson’s Knowledge argument and this knowledge argumentabout the self or subject of experiences. In particular, he suggeststhat each of these arguments is best understood as supporting a claimabout the limits of objectivism (seeSection 4.9: bothsubjects of experiences and the phenomenal character of experienceselude any objective description of reality.

A number of thinkers have explored parallels between the KnowledgeArgument and arguments in meta-ethics. One way to do this is to layout a challenge for meta-ethical naturalism, for instance by askingwhether a person who knew all the natural facts could thereby know allthe moral facts (Pelczar 2009; Yetter-Chappell & Yetter-Chappell2013; Blaesi 2022). In response to this challenge, Yetter-Chappell& Yetter-Chappell outline a Moral Concepts Strategy on which moraland natural concepts are isolated from each other, even though moralproperties just are natural properties (2013, 874–876). Pelczar offersan alternative response, explaining how a person can gain moralknowledge by entering into new relations with moral facts (seeSection 4.5). Blaesi uses thechallenge to naturalism to construct aparody of a widely-known argument for panpsychism, and argues thatthis meta-ethical parody threatens to reduce the argument forpanpsychism to absurdity.

Finally, Saad (2016) has used variations of the Mary thoughtexperiment to develop a parallel knowledge argument against certainkinds of dualism. Saad distinguishes between a number of differentfeatures of phenomenal states, including qualities (e.g., colours) andawareness of these qualities. He argues that even if Mary knew all thephysical and qualitative truths (truths about qualities), she wouldnot be able to work out all the truths about awareness (2016,2370–2371). Therefore, a dualism which holds that physical facts andfacts about qualities determine all other truths would be mistaken, inmuch the same way that the original knowledge argument is supposed toundermine physicalism.

7. Concluding Remark

The appropriate evaluation of the knowledge argument remainscontroversial. The acceptability of its second premise P2 (Mary lacksfactual knowledge before release) and of the inferences from P1 (Maryhas complete physical knowledge before release) to C1 (Mary knows allthe physical facts) and from P2 to C2 (Mary does not know some factsbefore release) depend on quite technical and controversial issuesabout (a) the appropriate theory of property concepts and theirrelation to the properties they express and (b) the appropriate theoryof belief content. It is therefore safe to predict that the discussionabout the knowledge argument will not come to an end in the nearfuture.

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