Auditory perception raises a host of challenging philosophical questions. What do we hear? What are the objects of hearing? What is the content and phenomenology of audition? Is hearing spatial? How does audition differ from vision and other sense modalities? How doesthe perception of sounds differ from that of colors and ordinary objects? This entry presents the main debates in this developing areaand discusses promising avenues for future inquiry. It discusses the motivation for exploring non-visual modalities, how audition bears ontheorizing about perception, and questions concerning the objects, contents, varieties, and bounds of auditory perception.

• 1. Other Modalities and the Philosophy of Perception
• 2. The Objects of Auditory Perception
  • 2.1 Sounds
    • 2.1.1 Private or Public?
    • 2.1.2 Proximal or Distal?
    • 2.1.3 Properties or Individuals?
    • 2.1.4 Objects or Events?
  • 2.2 Auditory Objects
    • 2.2.1 Object Perception in Audition
    • 2.2.2 What is an Auditory Object?
  • 2.3 Sound Sources
    • 2.3.1 Do Humans Hear Sound Sources?
    • 2.3.2 The Mereology of Sounds and Sources
• 3. The Contents of Auditory Perception
  • 3.1 Spatial Hearing
    • 3.1.1 Skepticism about Spatial Hearing
    • 3.1.2 Strawson and the Purely Auditory Experience
    • 3.1.3 Does Audition Have Spatial Structure?
    • 3.1.4 How Spatial Audition Differs from Spatial Vision
  • 3.2 Audible Qualities
    • 3.2.1 Sounds and Colors
    • 3.2.2 Pitch, Timbre, and Loudness
    • 3.2.3 Is Pitch Physical?
    • 3.3.4 Disanalogies with Color
    • 3.3.5 Analogies with Color
• 4. Varieties of Auditory Perception
  • 4.1 Musical Listening
    • 4.1.1 Acousmatic Experience
    • 4.1.2 Acousmatic Listening as Attention
  • 4.2 Speech Perception
    • 4.2.1 Is Speech Special?
    • 4.2.2 The Objects of Speech Perception
    • 4.2.3 The Contents of Speech Perception
    • 4.3.4 Is Speech Perception Auditory?
    • Supplement on Speech Perception
  • 4.3 Crossmodal Influences
    • 4.3.1 Crossmodal Illusions
    • 4.3.2 Causal or Constitutive?
    • 4.3.3 Multimodality in Perception
• 5. Conclusion and Future Directions
• Bibliography
• Academic Tools
• Other Internet Resources
• Related Entries

1. Other Modalities and the Philosophy of Perception

The philosophy of sounds and auditory perception is one emerging areaof the philosophy of perception that reaches beyond vision for insights about the nature, objects, contents, and varieties of perception. This entry characterizes central issues in the philosophy of auditory perception, many of which bear upon theorizing about perception more generally, and it mentions outstanding questions and promising future areas for inquiry in this developing literature. Before beginning the substantive discussion of audition itself, it isworthwhile to discuss the motivation and rationale for this kind of work.

Philosophical thinking about perception has focused predominantly onvision. The philosophical puzzle of perception and its proposedsolutions have been shaped by a concern for visual experience andvisual illusions. Questions and proposals about the nature ofperceptual content have been framed and evaluated in visual terms, anddetailed accounts of what we perceive frequently address just thevisual case. Vision informs our understanding of perception'sepistemological role and of its role in guiding action. It is not agreat exaggeration to say that much of the philosophy of perceptiontranslates roughly as philosophy of visual perception.

Recently, however, other perceptual modalities have attractedattention. In addition to auditory perception and the experience ofsound, touch and tactile awareness have generated philosophicalinterest concerning, for instance, the tactile and proprioceptiveexperience of space, the objects of touch, whether contact is requiredfor touch, and whether distinct modalities detect pressure, heat, andpain (see, e.g., O'Shaughnessy 1989, Martin 1993, Scott 2001,Fulkerson 2013). The unique phenomenology of olfaction and smells hasbeen used to argue that vision is atypical in supporting thetransparency of perceptual experience (Lycan 2000, 282; cf. Batty2010) and that perceptual objectivity does not require spatiality(Smith 2002, ch 5). Lycan (2000) even suggests that the philosophy ofperception would have taken a different course had it focused uponolfaction instead of vision (see also Batty 2011).

This kind of work is philosophically interesting in its own right.But it is also worthwhile because theorizing about perception commonlyaims to address general questions about perception, rather thanconcerns specific to vision. Hope for a comprehensive and generalunderstanding of perception rests upon extending and testing claims,arguments, and theories beyond vision. One might view work onnon-visual modalities as filling out the particulars required for athoroughly detailed account of perceiving that applies not just tovision but across the modalities. At least three approaches might beadopted, with potential for increasingly revisionist outcomes.

First, one might take work on non-visual modalities astranslating what we have learned from the visual case intoterms that apply to other modalities. This approach is relativelyconservative. It assumes that vision is representative or paradigmaticand that we have a good understanding of perception that is derivedfrom the case of vision. One example of this kind of approach would beto develop an account of the representational content of auditoryexperience.

Second, considering other modalities mightextend ourvision-based understanding of perception. Non-visual cases might drawattention to new kinds of phenomena that are missing from or notsalient in vision. If so, a vision-based account of perception issatisfactory as far as it goes, but it leaves out critical pieces. Forexample, speech perception, multimodal perception, and flavorperception might involve novel kinds of perceptual phenomena absentfrom the visual case.

Third, considering other modalities mightchallengevision-based claims about perception. If falsifying evidence isdiscovered in non-visual cases, then theorizing beyond vision mayforce revision of general claims about perception that are supportedby vision. For example, if olfactory experience is not diaphanous, thetransparency thesis for perceptual experience fails.

Finally, we might attempt to determine whether anyunifiedaccount exists that applies generally to all of the perceptualmodalities. We can ask this question either at the level of quitespecific claims, such as those concerning the objects of perception orthe nature and structure of content. We can ask it about therelationships among perceiving, believing, and acting. Or we can askit about the general theory of necessary and sufficient conditions forperceiving. Some philosophers, impressed by their findings concerningnon-visual modalities, express skepticism whether a unified theoryexists (e.g., Martin 1992).

Whatever the approach, extending our knowledge about perception beyondthe visual requires systematic attention to individual modalities aswell as careful accounting in order to determine how the results bearon general questions about perception. Whatever the outcome, auditionis a rich new subject matter in its own right, and developing thissubject matter is crucial to our overall understanding ofperception.

2. The Objects of Auditory Perception

What do we hear? One way to address this question concerns theobjects of auditory perception.

2.1 Sounds

In the first instance, it is plausible thatsounds are what wehear. Sounds aredirect orimmediate objects of auditoryperception.

What are sounds? Sounds traditionally have been counted with colors,smells, and tastes assecondary,sensible, orsensoryqualities (see, e.g., Locke 1689/1975, Pasnau1999, 2000). Recently, however, it has been proposed that sounds areindividuals to which sensible features are attributed. Inparticular, several philosophers recently have proposed that soundsare public, distally-located, event-like individuals (Casati and Dokic1994, 2005, O'Callaghan 2007, Matthen 2010).

Four questions about audition's objects define the debate and constrain theories of sound (see also the entry onsounds, for extensive discussion).

2.1.1 Private or Public?

Are sounds private or public? Maclachlan (1989) argues that the soundswe hear aresensations (rather than, for instance, the pressurewaves that cause auditory experiences). Such sensations are internaland private, and we experience them directly, or without apparentmediation. On Maclachlan's account, we hear the ordinary things andhappenings that are thesources of sounds onlyindirectly, by means of inference from auditory data.

Maclachlan's story is noteworthy partly because he uses hearingand sounds to motivate a general claim about perception. He claimsthat what seems perfectly intuitive and obvious in the case of soundsand hearing—that something other than material objects are thedirect objects of hearing; that the direct objects of audition areinternal; and that we indirectly hear things in the world by hearingtheir sounds—helps us to discover what is true of allperception. According to Maclachlan, for instance, seeing involvesdirect awareness of sensations of patterns of light, while surfacesand ordinary objects figure only indirectly and thanks to inferenceamong the intentional objects of sight. The case of sounds andaudition is important because it reveals that perceiving involvesawareness of sensations in the first instance, and of the externalworld only indirectly.

Maclachlan's description of sounds and auditory experience has someattractions. First, sounds are among the things we hear. And soundsare among the direct or immediate objects of audition in therelatively innocuous sense that hearing a sound does not seem torequire hearing as of something else. Hearing a collision, on theother hand, may seem to require awareness as of a sound. Furthermore,sounds are unlike the ordinary material objects (e.g., bottles andstaplers) we see. You cannot reach out and grab a sound, or determineits temperature. Instead, sounds may strike us as byproducts oreffects of such ordinary things and their transactions. Sounds resultfrom activities or interactions of material bodies and thus areexperienced as distinct or independent from them (cf. Nudds2001). Nevertheless, audition does afford some variety of awareness ofthe sources of sounds, or at least provides information aboutthem.

However, the claim that sounds are sensations is unattractive. Goodreasons suggest that sounds are public rather than private, even ifsounds are not identical with ordinary objects and events such asclothespins and collisions. Suppose I am near the stage in a halllistening to some music, and that I have a headache. It is aconfusion to think you could feel my headache, but I assume you hearthe sounds I hear. Suppose I move to the back of the hall, and theheadache then gets better. My experience of the sounds of the musicdiffers once I am at the back of the room, and my experience of theheadache differs. The sound of the music itself need not differ (themusicians could make the same sounds), but the headache itselfchanges. The sounds can continue once I leave the room, but if I stopexperiencing the headache, it is gone. Moreover, the notion of anunfelt headaches is puzzling, but it makes good sense to say that atree makes a sound when it falls in the woods without beingheard. Finally, tinnitus, or ringing of the ears, is an illusory orhallucinatory experience as of a sound, but there are no illusoryheadaches.

This suggests that audition does not providespecial reasons tobelieve that the objects of perception are private sensations.Sounds, construed as objects of auditory perception, plausibly inhabitthe public world. (See the section3.1 Spatial Hearing for further discussion.)

2.1.2 Proximal or Distal?

Are sounds proximal or distal? The customary science-based view holdsthat sounds are pressure waves that travel through a medium. On thisaccount, sounds are caused by objects and events such as collisions,and sounds cause auditory experiences. However, sounds are notauditorily experienced to travel through the surrounding medium aswaves do. Thus, if sounds are waves, then the sounds we hear may beproximal, located at the ear of the hearer.

Alternatively, some have argued that audition presents sounds asbeing located in some direction at a distance (Pasnau 1999,O'Callaghan 2007, ch. 3, 2010). On such an account, sounds commonlyappear auditorily to be in the neighborhood of their sources andthereby furnish useful information about the locations of thosesources. The sound of the drumming across the street seems to comefrom across the street but does not seem audibly to travel. Whensounds do appear to fill a room, sound seems located allaround. Sounds that seem to “bounce” around a room appearintermittently at different locations rather than as travelingcontinuously from place to place. Experiencing a missile-like soundspeeding towards your ears illustrates the contrast with ordinaryhearing (O'Callaghan 2007, 35). Sounds, according to this conception,ordinarily appear to have distal locations and to remain stationaryrelative to their sources.

If sounds are not usually experienced to travel, then unless auditoryexperience is illusory with respect to the apparent locations ofsounds, sounds themselves do not travel. Sounds thus are not identicalwith and do not supervene upon the waves, since waves travel (Pasnau1999). Several philosophers have argued on these and related groundsthat sounds are located distally, near their sources (Pasnau 1999,Casati and Dokic 2005, O'Callaghan 2007). On this view, pressure wavesbear information about sounds and are the proximal causes of auditoryexperiences, but are not identical with sounds.

One might object by resisting the phenomenological claim thatwe experience sounds as distally located, for instance by suggesting that audition is aspatial, or that audition is spatial but soundsources rather than sounds are auditorily localized (see section3.1 Spatial Hearing for further discussion). Or, one might accept some measure of illusion. Another possibility is that we experience only a smallsubset of the locations sounds occupy during their lifetimes (forinstance, while at their sources), and simply fail to experience wherethey are at other times. This avoids ascribing illusion. Finally,Fowler (2013) argues indirectly on the basis of echoes against distaltheories of sound.

2.1.3 Properties or Individuals?

Are sounds properties or individuals? Among both proximal and distal theories, disagreement exists concerning the ontological category to which sounds belong. Philosophers traditionally have understood sounds as properties—either as sensible or secondary qualities,or as the categorical or physical properties that ground powers to affect subjects. Commonly, sounds are attributed to the medium that intervenes between sources and perceivers. More recently, however, some distal theorists have argued that sounds are properties of what we ordinarily understand as sound sources—bells and whistleshave orpossess rather thanmake orproduce sounds. Pasnau (1999), for instance, claims that sounds are transient properties that are identical with or supervene upon vibrations of objects. Kulvicki (2008) argues against transiencein an attempt to subsume sounds to the model of colors, and claims that sounds are persistent, stable dispositional properties of objects to vibrate in response to being “thwacked”. He distinguishes “having” a stable sound from “making” a sound on some occasion (manifesting the stabledisposition). This account implies that sounds sometimes make sounds they do not have, and that they have sounds when silent.

One might ask property theorists whether events such as collisions and strummings, rather than objects, bear sounds. A more serious challenge comes from those who argue that sounds areindividuals rather than properties. Several arguments support this understanding. First, empirical work onauditory scene analysis suggests that one primary task of audition is to carve up the acoustic scene into distinct sounds, each of which may possessits own pitch, timbre, and loudness (Bregman 1990). Multiple distinctsounds with different audible attributes can be heard simultaneously.An analog of Jackson's (1977, see also Clark 2000)many properties problem thus arises for audition since feature awareness alone cannot explain the bundling or grouping of audible attributes into distinct sounds. Such bundling or grouping of audiblefeatures suggests that sounds are perceptible individuals to which these features are attributed.

Furthermore, the temporal characteristics of experienced sounds suggest that sounds are not simple qualities. Sounds audibly seem to persist through time and to survive change. A particular sound, such as that of an emergency siren, might begin high-pitched and loud and end low-pitched and soft. This suggests that sounds are individuals that bear different features at different times, rather than sensiblequalities (cf. Cohen 2009).

Several responses to these arguments are available. One might argue that sounds are complex properties, such as pitch-timbre-loudness complexes, instantiated at a time. To account for feature binding, one might hold that such complex properties are ascribed to ordinary objects such as bells and whistles. Or, one might hold that they are particularized properties, such as tropes. To accommodate sounds thatsurvive change through time, a property account could hold that sounds are yet more complex properties that have patterns of change built into their identity conditions. Any such view differs a great deal from the familiar secondary or sensible quality view pioneered by Locke. Pitch, timbre, and loudness are better candidates for simple sensible features in audition (see section3.2 Audible Qualities).

2.1.4 Objects or Events?

If sounds are individuals, are they object-like or event-likeindividuals? Intuitively, the material objects we see are capable ofexisting wholly at any given moment, and all that is required toperceptually recognize such individuals is present at a moment. On theother hand, event-like individuals occupy time and need not existwholly at any given moment. Their individuation and recognitionfrequently appeal to patterns of features over time. Event-likeindividuals intuitively comprise temporal parts, while object-likeindividuals intuitively do not. The issue here is not the truth ofendurantism or perdurantism as an account of the persistence ofobjects or events. Instead, the issue concerns a difference in how weperceptually individuate, experience, and recognize individuals.

No contemporary philosopher has yet claimed that sounds are objectsin the ordinary sense. Those who argue that sounds areindividuals commonly point out that sounds not only persist andsurvive change (as do ordinary material objects), but alsorequire time to occur or unfold. It is difficult to imagine aninstantaneous sound, or one that lacks duration. Sounds are notcommonly treated as existing wholly at a given moment during theirduration. Indeed, the identities of many common sounds are tied topatterns of change in qualities through time. The sound of anambulance siren differs from that of a police siren precisely becausethe two differ in patterns of qualitative change through time. Thesound of the spoken word ‘team’ differs from that of‘meat’ because each instantiates a common set of audiblequalities in a different temporal pattern. These considerationssupport the view that sounds are event-like individuals (see Casatiand Dokic 1994, 2005, Scruton 1997, O'Callaghan 2007, Matthen2010).

This may bear on debates about persistence in the followingway. Differences in the intuitive plausibility of endurantism andperdurantism may be grounded in facts about perception. In particular,vision may treat objects as persisting by enduring or being whollypresent at each time at which they exist, while audition may treat itsobjects as persisting by perduring or having temporal parts. This maystem from differences in perceptual organization. For instance,displaying a visible property profile at a time may suffice for beinga visual object of a given sort, while being an audible object of agiven sort may require displaying an audible property profile overtime.

2.2 Auditory Objects

Though most philosophers construe sounds either as properties or asevent-like individuals (see section2.1 Sounds),much recent discussion among psychologists has concernedauditoryobjects (see, e.g., Kubovy and Van Valkenburg 2001, Griffiths andWarren 2004). The target of such discussion is not simply audition'sintentional objects orproper (specific to audition)objects. The intended analogy is withvisual objects. Talk ofauditory objects gestures at the visual processes involved inperceptually discriminating, attentively tracking, recognizing andcategorizing ordinary material objects. What justifies talk ofobject perception in audition?

2.2.1 Object Perception in Audition

First of all, humans typically do not auditorily perceivethree-dimensional, bounded material objectsas such, though itis plausible to think we visually perceive them. Hearing does notresolve the edges, boundaries, and filled volumes in space that I see,and I do not hear audible items to complete spatially behind occludersas do visible surfaces of objects. If perceiving a three-dimensionalobject requires awareness of its edges, boundaries, and extension,perhaps in order to discriminate it from its surroundings, humanstypically do not auditorily perceive such objects.

Nevertheless, striking and illuminating parallels do exist between the perceptual processes and experiences that take place in vision and audition. Such parallels may warrant talk of object perception ina more general sense that is common to both vision and audition (and perhaps touch, though I don't pursue that here).

Perceiving objects requires parsing a perceptual scene into distinctunits that one can attend to and distinguish from each otherand from a background. In vision, bounded, cohesive collections ofsurfaces that are extended in space and that persist through time playthis role (see, e.g., Spelke 1990, Nakayama et al. 1995, Leslie etal. 1998, Scholl 2001, Matthen 2005). In audition, as in vision,multiple distinct perceptible individuals might exist simultaneously,and each might persist and survive change (see the discussion ofauditory scene analysis in section2.1 Sounds).A critical difference, however, is that while vision's objects areextended in space, and are individuated and recognized primarily invirtue of spatial characteristics, audible individuals are extended intime, and are perceptually individuated and recognized primarily invirtue of pitch and temporal characteristics (see, e.g., Bregman 1990,Kubovy and Van Valkenburg 2001). For instance, audible individualshave temporal edges and boundaries, and boundary elements can belongonly to a single audible individual. They also are susceptible tofigure-ground effects over time. One can, for instance, shiftattention among continuous audible individuals that differ inpitch. Furthermore, they are susceptible to completion effects overtime in much the same way that visible objects are perceptuallycompleted in space. Seeing a single visible region to continue behinda barrier is analogous to hearing a sound stream to continue throughmasking noise, which may take place even when there is nocorresponding signal (Bregman 1990, 28). Finally, multiple distinct,discrete audible individuals, such as the temporally bounded notes ina tune, can form audiblestreams that comprise a singleperceptible unit. Such streams are subject to figure-ground shifts,and, like collections of surfaces, they can be attentively trackedthrough changes to their features and to one's perspective. Thoughsuch complex audible individuals include sounds, they comprisetemporally unified collections of sounds and silence that areanalogous to spatially complex visible objects, such as tractors.

Such audible individuals are temporally extended and bounded, serve asthe locus for auditory attention, prompt completion effects, and aresubject to figure-ground distinctions in pitch space. For thesereasons, the auditory processes involved in their perception parallelthose involved in the visual perception of ordinary three-dimensionalobjects. The parallels suggest a shared sense in which vision andaudition involve a more general form of object perception (see, e.g.,Kubovy and Van Valkenburg 2001, Scholl 2001, Griffiths and Warren2004, O'Callaghan 2008a, Matthen 2010).

2.2.2 What is an Auditory Object?

What is the shared sense in which both visible and audible individualsdeserve to be called ‘objects’? Kubovy and Van Valkenburg(2001, 2003) define objecthood in terms of figure-ground segregation,which requires perceptual grouping. They propose thetheory ofindispensable attributes as an account of the necessary conditionson perceptual grouping (see also Kubovy 1981). Indispensableattributes for a modality are those without which perceptualnumerosity is impossible. They claim that while space and time areindispensable attributes for vision (and color is not), pitch and timeare indispensable attributes for auditory objects. Though they aremore skeptical about whether audition parallels vision, Griffiths andWarren (2004) sympathize with a figure-ground characterization butsuggest a working notion of an auditory object defined in terms of“an acoustic experience that produces a two-dimensional imagewith frequency and time dimensions” (Griffiths and Warren 2004,891).

O'Callaghan (2008a) proposes that both visible and audible objects are mereologically complex individuals, though their mereology differs in noteworthy respects. While vision's objects possess a spatial mereology and are individuated and tracked in terms of spatial features, audition's objects have a temporal mereology and are individuated and tracked in terms of both pitch and temporal characteristics. Discussion of auditory objects thus draws attention to two roles that space plays in vision. First, there is the role of space in determining the structure internal to visible objects, whichfacilitates identifying and recognizing visible objects. Second, space serves as the external structure among visible objects, and is critical in distinguishing objects from each other. In audition, timeplays a role similar to space in vision in determining the structure internal to auditory objects. Pitch, on the other hand, serves as an external structural framework, along with space, that helps to distinguish among audible individuals.

Why is it useful to perceive such individuals in audition? One promising account is that they provide useful information about the happenings that produce sounds. Carving the acoustic world into mereologically complex individuals informs us about what is going on in the extra-acoustic environment. It provides ecologically significant information about what the furniture is doing, rather than just how it is arranged. It is one thing to perceive a tree; it is another to hear that it is falling behind you.

Discussion of auditory objects and accounts of their nature andperception is new among philosophers (see, e.g., O'Callaghan 2008a,and essays in Bullot and Egré 2010, including Matthen 2010, Nudds2010). This area is ripe for philosophical contributions.

2.3 Sound Sources

Sounds are among the objects of audition. Plausibly, so are complex,temporally extended individuals composed of sounds. Do we hearanything else? Reflectionsuggests we hear things beyond soundsand sound complexes. In hearing sounds, one may seem to experience thebackfiring of the car or the banging of the drum. One might hold thata primary part of audition's function is to revealsoundsources, the things and happenings that make sounds.

2.3.1 Do Humans Hear Sound Sources?

If sounds were internal sensations or sense-data, then, as Maclachlan(1989) observes, we would hear sound sources only indirectly, in an epistemological sense, perhaps thanks to something akin to inference.Acquiring beliefs about the environment would require mediation by propositions connecting experienced internal sounds with environmental causes.

If, however, sounds are properties attributed either to ordinary objects, as Pasnau (1999) and Kulvicki (2008) hold, or to events, then hearing a tuba or the playing of a tuba might only require hearing its sounds. Perceptually ascribing such audible attributes totheir sources might ground epistemically unmediated awareness of tubas or their playings.

However, the individuals to which audible attributes are perceptuallyattributed need not be identical with ordinary objects orevents. Instead, audible attributes may belong in the first instanceto sounds. Sounds plausibly are distinct from ordinary orextra-acoustic individuals (O'Callaghan 2007, 2011). Suppose then thatone could not hear an ordinary object without there existing anaudible sound, as well as that sounds can mislead about their sources(it might sound like drumming but be hammering).

Given this, forming beliefs about ordinary things and happeningsconnected with sounds might seem to require inference, association, orsome otherwise cognitive process, and so awareness of a sound sourcemight appear to always involve more than perceptual awareness.According to such an account, awareness of environmental things andhappenings thanks to audition is epistemically mediated by awarenessas of sounds and auditory objects, but does not itself constituteauditory perceptual awareness as of those things and happenings. Youare inclined to think you hear the source because your representing orbeing aware of it co-occurs with, but is no more than a downstreamconsequence triggered by, your auditory experience.

Such an account is not wholly satisfactory. First, the phenomenologyof audition suggests something stronger than indirect, epistemicallymediated awareness of things such as collisions or guitar strummingsor lions roaring. Reflection suggests auditory awarenessas ofcollisions, strummings, and lions. Second, the capacity to referdemonstratively to such things and events on auditory grounds alsosuggests genuine perceptual awareness of them. Third, we commonlyperceptually individuate sounds in terms of their apparent sources,and our taxonomy reflects this. “What did you hear?”“I heard paper ripping,” or, “The sound of adripping faucet.” We distinguish two quite similar rattles oncewe hear one as of a muffler clamp and the other as of a loosefender. Furthermore, characterizing certain audible features andexplaining perceptual constancy effects for them requires appeal tosound sources. Handel says of timbre: “At this point, no knownacoustic invariants can be said to underlie timbre... The cues thatdetermine timbre quality are interdependent because all are determinedby the method of sound production and the physical construction of theinstrument” (Handel 1995, 441). Explaining loudnessconstancy—why moving to the back of the room does not change howloudly the lecturer seems to speak—appeals to facts about thesources of sounds (Zahorik and Wightman 2001). Auditory processingproceeds in accordance with natural constraints concerningcharacteristics of sound sources, and information concerning sourcesshapes how auditory experiences are organized. This is to say thatprocesses responsible for auditory experience proceed as if acousticinformation is information about sound sources. Finally,audition-guided action supports the claim that we hear such things andevents. Turning to look toward the source of a sound or ducking out ofthe way of something we hear to be approaching—behaviors guidedby auditory experience—would make little sense if we heard onlysounds. In the first place, these reasons ground a case for thinkingthat auditory perceptual experience does not strictly end with soundsand auditory objects. In particular, awareness as of a source, even ifdependent upon awareness as of a sound, may be constitutive of one'sauditory perceptual experience.

The main barrier to an alternative is that the relation between soundsand ordinary things or happenings is commonly understood ascausal. Awareness as of an effect does not itself typicallyfurnish epistemically unmediated awareness of its cause. Seeing smokeisn't seeing fire. The right sort of dependence betweencharacteristics of the experience and the cause is not apparent, andawareness as of an effect does not by itself ground perceptualdemonstratives concerning the cause. The metaphysical indirectness ofthe causal relation appears to block epistemic directness (seeO'Callaghan 2011a for further discussion).

2.3.2 The Mereology of Sounds and Sources

Is there another explanatory route? Suppose that instead of a causalrelation, we understand the relationship between sounds and sourcesmereologically, or as one of part to whole (see O'Callaghan2011a). Parthood frequently does ground perceptual awareness. Forinstance, seeing distinct parts of a surface interrupted by anoccluder leads to perceptual experience as of a single surface(imagine seeing a dog behind a picket fence). Seeing the facingsurfaces of a cube affords awareness as of a cube, and we canattentively track that same cube as it rotates and reveals differentsurfaces. Suppose, then, that a sound is an event-like individual(recall, property accounts escape the worry). This event is part of amore encompassing event, such as a collision or the playing of atrumpet, that occurs in the environment and that includes thesound. So, the typical horse race includes the sounds, and you mightauditorily perceive the racing in hearing some of its proper parts:the sounds. More specifically, you may hear the galloping thanks tohearing the sounds it includes. You may fail to hear certain parts ofthe racing event, such as the jockey's glance back after crossing thewire, but you also fail to see parts of the race, such as the misstepof the horse in second place. If the sounds are akin to the audible“profile” of the event, analogous to the visible surfacesof objects and visible parts of events, you might then enjoy auditoryawareness as of the galloping of the horses in virtue of yourawareness as of the sounds of the hooves. The sound is not identicalwith the galloping, and it is not just a property or a causalbyproduct of the galloping. It is a part of a particular event ofgalloping. The metaphysical relation of part to whole, in contrast tothat between effect and cause, might ground the sort of epistemicallyunmediated awareness of interest (cf. Nakayama et al. 1995,Bermúdez 2000, Noë 2004). Auditory perceptual awareness asof the whole may occur thanks to experiencing the part.

One objection is that this mereological account of the relationbetween sounds and sources cannot account for hearing ordinaryobjects by hearing their sounds. You could not strictly hear atuba by hearing its sound because a tuba is not an event of which asound is a part. However, the sound is part of the event of playingthe tuba, and the tuba is a participant in that playing. So, thoughyou are not aware as of a tuba, you are aware as of an event thatinvolves a tuba. That perhaps is enough to explain talk of hearingtubas and to assuage the worry.

Another more serious objection contends that the events we seem tohear are ones that do not constitutively involve sounds or that mighthave taken place without sounds. For instance, we hear the collision,but the collision is something that could have occurred in a vacuumand not made a sound. If so, the collision and the sound differ andthe collision does not strictly include a sound. The collisiontherefore must have made the sound as a causal byproduct. Thissuggests that, strictly speaking, you could not hear that verycollision event (since it causes the sound). The best response is tobite the bullet and accept that events that do occur or that couldoccur in vacuums cannot be heard since they include no sounds. This isnot so bad, since you could hear a different, more encompassing eventthat includes a sound (along with a collision). Alternatively, onemight say the very same event that occurs in a vacuum also could occurin air, but that it would have involved a sound had it occurred inair. In that case, one can only hear such events when they occur inair and include a sound. The choice depends upon one's metaphysics ofevents. In either case, it seems reasonable that token events that donot include sounds are inaudible.

Casati et al. (2013) sidestep some of these concerns by rejectingthe distinction between sounds and events that are typicallyunderstood as sound sources. They propose to “Ockhamize”the “event sources” of sounds by identifying sounds withevents such as collisions and vibrations. The sound just is thecollision or the vibrating, whether or not it occurs in air. Thisaccount implies that sounds could exist in vacuums.

What hinges on the debate about hearing sources? The first upshot is epistemological and concerns the nature of the justification for empirical beliefs grounded in perceptual experience. The evidential status of beliefs about what one perceptually experiences differs from that of beliefs about what is causally responsible for what one perceptually experiences. The second upshot concerns the relation between audition and certain actions. If we hear only sounds and auditory objects, what appears to be effortless, auditorily guided action to avoid or orient toward sound sources requires another explanation (because sounds are invisible and usually do no harm). Finally, it impacts how we understand the adaptive significance of audition. Did audition evolve so as to furnish awareness of sounds alone, while leaving their environmental significance to extra-perceptual cognition, or did it evolve so as to furnish perceptual responsiveness to the sources of sounds?

3. The Contents of Auditory Perception

Another way to address the question, “What do we hear?” concerns thecontents of auditory perception. Two topics are especially noteworthy in the context of related debates about vision and its contents. The first concerns the whether audition has spatial content. The second concerns the perception of audible qualities.

3.1 Spatial Hearing

One topic where the contrast between vision and audition has been thought to be particularly philosophically significant concerns space. Vision is a robustly spatial perceptual modality. Vision furnishes awareness of space and spatial features. Some claim vision has an inherently spatial structure, or, further, that vision's spatial structure is a necessary condition on being visually aware ofthingsas independent from oneself.

Hearing also provides information about space—humans learn about space on the basis of hearing. If audition represents space or spatial features, a natural account of being so informed follows. We might form beliefs about spatial features of environments on the basis of auditory perceptual experiences simply by accepting or endorsing what is apparent in those experiences.

But learning about spatial features on the basis of audition andaudition's bearing information about space both are consistent withentirely aspatial auditory phenomenology. For instance, volume mightbear information about distance, and differences in volume at the twoears might bear information about direction. In that case, auditionbears information about space, and learning about space on the basisof audition is possible, but it does not follow that auditoryexperience is spatial or that audition represents space.

3.1.1 Skepticism about Spatial Audition

Notably, a tradition of skepticism about audition's spatiality existsin philosophy. Certainly, our capacity to glean information about space is less acute in audition than in vision. Vision reveals fine-grained spatial details, such as patterns and textures, that audition cannot convey. But philosophers who are skeptical about spatial audition are not just concerned about a difference in spatialacuity between audition and vision. Malpas says of the expression, ‘the location of sound’:

I do not mean by ‘location’‘locality’, but ‘the act of locating’, and by‘the act of locating’ I do not mean ‘the act ofestablishing in a place’, but ‘the act of discovering theplace of’. Even so ‘location’ is misleading, becauseit implies that there is such a thing as discovering the place ofsounds. Since sounds do not have places there is no such act. (Malpas1965, 131)

O'Shaughnessy states, “…We absolutely never immediately perceive sounds to beat any place. (Inference from auditory data being another thing)” (O'Shaughnessy 2002, 446). The claimis that, in contrast to visible objects, audible sounds are not experienced as having locations. Rather, we determine the places of sounds and sources from acoustic features, such as loudnessand interaural differences, that bear information about distance and direction. We do not auditorily experience spatial features.

This debate, and the purported contrast between vision and audition,has consequences for perceptual theorizing. One route to theconclusion that hearing sounds is auditory awareness of sensationsinvolves denying that audition satisfies spatial prerequisites onexperiencing sounds as objective or public. For instance, Maclachlan(1989) claims that audition's phenomenology—in particular, itsaspatial phenomenology—provides reasons to think sounds aresensations. Comparing sounds with pains, which we readily recognize assensations, he says, “[A]lthough the sounds we hear are just asmuch effects produced in us as are the pains produced by pins andmosquitoes,there is no variety in the location of theseeffects [the sounds]. Because of the lack of contrast, we are noteven aware that the sounds we hear are bodily sensations”(Maclachlan 1989, 31, my emphasis). Maclachlan means that, in contrasteven to the case of pains, which are felt at different bodilylocations, sounds are not experienced to be at differing locations,and so we are not even inclined to recognize that they are bodilysensations. Maclachlan then suggests that weassociate soundswith things and happenings outside the body rather than appreciatethat they are effects in us. Given the lack of spatialvariation among experienced sounds, we projectively associatesounds with distal sources. This explanation assumes that experiencedsounds exhibit no audibly apparent spatial variation: sounds seemlocated at the ears or lack apparent location altogether. Denying thatauditory experiences present sounds at varying locations beyond theears invites difficulty in finding a place for sounds in the world. Ifaudition is wholly aspatial, this may encourage a retreat to the viewthat sounds lack locations outside the mind.

This kind of strategy has companions and precursors. Lycan's suggestion that olfactory experiences are apparent as modifications of one's own consciousness depends heavily on the aspatial phenomenology of olfactory experience (Lycan 2000, 278–82). Each recalls the Kantian claim that objectivity requires space, or that grasping something as independent from oneself requires the experience of space, a version of which is deployed by Strawson (1959, ch 2) in his famous discussion of sounds.

Two lines of response are open. The first appeals to the thrivingempirical research program in “spatial hearing” (see, e.g., Blauert 1997). Scientists aim to discover the cues and perceptual mechanisms that ground spatial audition, such as interaural time and level differences, secondary and reverberant signals, and head-related transfer functions. Audition clearly cannotmatch vision's singular acuity—vision's resolution limit is nearly two orders of magnitude better than audition's (Blauert 1997, 38–9). Nevertheless, this research strongly supports the claim that human subjects auditorily perceive such spatial characteristics as direction and distance.

Second, a number of philosophers have objected onphenomenological grounds. Audition, they argue, involves experiencing or perceptually representing such spatial characteristics as direction and distance (Pasnau 1999, Casati and Dokic 2005, Matthen 2005, O'Callaghan 2007, 2010). Introspection and performance support the claim that sounds or sound sources are inmany ordinary cases perceptually experienced as located in the environment at a distance in some direction. We hear the sound of theknocking over near the door; we hear footsteps approaching from behind and to the left; hearing sound to “fill” a room isitself a form of spatial hearing. Though hearing is more error prone than vision, we frequently do not need to work out the locations of sounds or sources—we simply hear them.

3.1.2 Strawson and the Purely Auditory Experience

A subtler form of skepticism about spatial audition aims just to block the requirements on objectivity. Strawson (1959) famously argues in Chapter 2 ofIndividuals that because auditory experience is notintrinsically spatial—spatial conceptshave no intrinsic auditory significance—a purely auditory experience would be non-spatial. It thus would not satisfy the requirements on non-solipsistic consciousness. Others have endorsed versions of Strawson's claim. “[T]he truth of a proposition to the effect that there is a sound at such-and-such a position must consist in this: if someone was to go to that position, he would havecertain auditory experiences,” states Evans (1980, 274).

The claim that audition is not intrinsically spatial admits at least two readings. First, since Strawson suggests that audition might inherit spatial content from other sense modalities such as vision ortouch, it could mean that auditiondepends for its spatial content upon that of other modalities. If, unlike vision and touch, audition's spatial capacities are parasitic upon those of other modalities, audition is spatial only thanks to its relations to otherintrinsically spatial modalities. Second, it might be understood as aclaim about theobjects of audition. Strawson indicates that sounds themselves are not intrinsically spatial. He says that although sounds have pitch, timbre, and loudness, they lack “intrinsic spatial characteristics” (1959, 65). Since these interpretations are not clearly distinguished by Strawson, it is helpful to consider his master argument.

Strawson claims that a purely auditory experience would be non-spatial. By “purely auditory experience” Strawson means an exclusively auditory experience, or an auditory experience in absence of experience associated with any other modality. Given the mechanisms of spatial hearing, however, it is empirically implausible that a normal acoustic environment with rich spatial cueswould fail to produce even a minimally spatial purely auditory experience. Even listening only to stereo headphones could produce a directional auditory experience. If any modalityin isolation ever could ground spatial experience, audition could. On the other hand, it does seem possible that there could be a non-spatial butimpoverished exclusively auditory experience if no binaural orother spatial cues were present. But similarly impoverished, non-spatial experiences seem possible for other modalities. Consider visually experiencing a uniform gray ganzfeld, or floating weightlessly in a uniformly warm bath. Neither provides the materialsfor spatial concepts, so neither differs from audition in this respect. One might contend that we therefore lack a good reason to think that,in contrast to a purely visual or tactile experience, a purely auditory experience would be an entirely non-spatial experience (see O'Callaghan 2010).

3.1.3 Does Audition Have Spatial Structure?

Nudds (2001) suggests another way to understand the claim, and interprets Strawson as making an observation about the internalstructure of audition:

When we see (or seem to see) something, we see it asoccupying or as located within a region of space; when we hear (orappear to hear) a sound we simply hear the sound, and we don'texperience it as standing in any relation to the space it may in factoccupy. (Nudds 2001, 213–14)

Audition, unlike vision, lacks a spatial structure or field, claims Nudds. A purely auditory experience thus would not comprise a spatialfield into which individuals independent from oneself might figure. Following an example from Martin (1992), Nudds argues that while vision involves awareness of unoccupied locations, audition does not involve awareness of regions of space as empty or unoccupied. Martin's example is seeing the space in the center of a ringas empty. In audition, Nudds claims, one never experiences a space as empty or unoccupied.

In response, one might simply deny a difference between vision andaudition on this count. If one can attend to a location near thecenter of the visible ring as empty, one can attend to the locationbetween the sounding alarm clock and the slamming door as a placewhere there is no audible sound—as acoustically empty space. Ofcourse, auditory space generally is less replete than visual space,but this is contingent. Consider seeing just a few stars flickering onand off against a dark sky. Since such an experience may have spatialstructure, and since it is analogous to audition, one might on thesegrounds defend the claim that audition has spatial structure.

3.1.4 How Spatial Audition Differs from Spatial Vision

What about the second way mentioned above to understand Strawson'sclaim? Though audition's status as intrinsically spatial may notdiffer from that of vision or touch, perhaps sounds are notintrinsically spatial. But without further argument, or a commitmentto a theory of sounds, it is difficult to state confidently theintrinsic features of sounds and thus whether they include spatialfeatures. If, for instance, wavelength is among a sound's intrinsicfeatures, sounds are intrinsically spatial.

Nonetheless, the claim might be that sounds, as they are perceptually experienced to be, lack intrinsic or non-relational spatial features. Roughly, independent from spatial relations to other sounds, experienced sounds seem to lackinternal spatial structure. That is why you cannot auditorily experience the empty space at the center of a sound or hear its edges. Interpreted as such—that sounds are not experienced or perceptually represented to have inherent spatial features—the claim is plausible (though consider diffuse or spread out sounds in contrast to focused or pinpoint sounds). It certainly marks an important difference from vision, whose objects frequently not only seem to have rich internal spatial structure, butalso are individuated in terms of inherent spatial features.

This difference, however, does not ground an argument that any purelyauditory experience is non-spatial or that sounds fail to satisfy therequirement on objectivity, since sounds' being experienced to haveinternal, intrinsic, or inherent spatial characteristics is necessaryneither for spatial auditory experience nor to experience sounds asobjective. Since sounds phenomenologically seem to be located in spaceand to bear extrinsic spatial relations to each other, auditoryexperience satisfies the requirements for objectivity, which need onlysecure the materials for a conception of a place for sounds to existwhen not experienced.

So, vision and audition differ with respect to space in two ways. First, vision's spatialacuity surpasses that of audition. Second, vision's objects are perceptually experienced to have richinternal spatial structure, and audition's are not. However, given the spatialcharacteristics evident in audition, such as direction and distance, the spatial status of audition presents no barrier to understanding its objects as perceiver-independent. The spatial aspects of auditoryphenomenology thus may fail to ground an argument to the conclusion that sounds are modifications of one's consciousness. If that is the case, then audition provides no special intuitive support for accounts on which private entities are the direct objects of perception.

3.2 Audible Qualities

3.2.1 Sounds and Colors

According to theories on which sounds are individuals, sounds are notsecondary or sensible qualities. But, humans hear audible qualities, such as pitch, loudness, and timbre, that are analogous to colors, tastes, and smells. Thus, familiar accounts of colors and other sensible attributes or secondary qualities might apply to the audiblequalities. For instance, pitches might be either dispositions to cause certain kinds of experiences in suitable subjects, the physicalor categorical bases of such dispositions, sensations or projected features of auditory experiences, or simple primitive properties of (actual or edenic) sounds.

Tradition suggests that the form of a philosophical account of visible qualities, such as color, and their perception applies to other sensible qualities, such as pitch, flavor, and smell, and theirperception. Thus, according to tradition, if dispositionalism, physicalism, projectivism, or primitivism about sensible qualities istrue for features associated with one modality, it is true for features associated with others. Despite tradition, we should be waryto accept that a theory of sensible qualities translates plausibly across the senses.

Debates about sensible qualities and their perception begin with concerns about whether sensible features can be identified with or reduced to any objective physical features. What follows has two aims. The first is to give a sense of how such debates might go in the case of audible qualities. The focus is on pitch, since pitch is often compared to color, and the case of color is well known. The second is to point out the most salient differences and similarities between the cases of color and pitch that impact the plausibility of arguments translated from one case to the other.

First, I consider two noteworthy arguments that are founded on aspectsof color perception. Each aims to establish that the colors weperceive cannot be identified with objective physicalfeatures. Neither argument transposes neatly to the case ofpitch. Thus, we should not assume arguments that are effective in thecase of color have equal force when applied to other sensiblequalities. Color perhaps is a uniquely difficult case.

Second, however, I discuss two respects in which pitch experience issimilar to color experience. It is instructive that these aspects ofpitch experience do raise difficulties for an objective physicalaccount of pitch that are familiar from the case of color.

3.2.2 Pitch, Timbre, and Loudness

What are pitch, timbre, and loudness? Pitch is a dimension alongwhich tones can be ordered according to apparent“height”. The pitch of fingernails scratching a blackboardgenerally is higher than that of thumping a washtub. Loudness can beglossed as the volume, intensity, or quantity of sound. A jet planemakes louder sounds than a model plane. Timbre is more difficult todescribe. Timbre is a quality in which sounds that share pitch andloudness might differ. So, a violin, a cello, and a piano all playingthe same note differ in timbre. Sometimes timbre is called “tonecolor”.

Physics and psychoacoustics show that properties including frequency,amplitude, and wave shape determine the audible qualities sounds(auditorily) appear to have. To simplify, take the case of pitch,since pitch often is compared to color. Not all sounds appear to havepitch. Some sounds appear to have pitch thanks to a simple,sinusoidal pattern of vibration at some frequency in an objector in the air. Some sounds appear pitched thanks to a complex patternof vibration that can be decomposed into sinusoidal constituents atmultiple frequencies, since any pattern of vibration can be analyzedas some combination of simple sinusoids. Sounds appear pitched,however, just when they have sinusoidal constituents, orpartials, that all are integer multiples of a commonfundamental frequency. Sounds with pitch thus correspond toregular orperiodic patterns of vibration that differ infundamental frequency and complexity. Simple sinusoids and complexwaveforms match in pitch (though they typically differ in timbre) whenthey share fundamental frequency, even when the complex tone lacks asinusoidal constituent at the fundamental frequency (the phenomenon ofthemissing fundamental).

3.2.3 Is Pitch Physical?

A straightforward account identifies pitch with periodicity (perhapswithin some range). Having pitch is being periodic (see O'Callaghan2007, ch. 6). Periodicity can be expressed in terms of fundamentalfrequency, so individual pitches are fundamental frequencies. This hasadvantages as an account of pitch. It captures the linear ordering ofpitches. It also explains the musical intervals, such as the octave,fifth, and fourth, for example, which are pitch relations that holdamong periodic tones. Musical intervals correspond to whole-numberratios between fundamental frequencies. Sounds that differ by anoctave have fundamental frequencies that stand in 1:2 ratios. Fifthsinvolve a 2:3 relationship, fourths are 3:4, and so on. This alsoallows us to revise the linear pitch ordering to accommodate theauditory sense in which tones that differ by an octave nonetheless arethe same pitch. If the pitch ordering is represented as a helix, uponwhich successive octave-related tones fall at a common angularposition, each full rotation represents doubling frequency.

Is the periodicity theory of pitch plausible as an account of the audible features we perceive when hearing sounds? If so, then objective physicalism about at least some sensible qualities might succeed.

3.2.4 Disanalogies with Color

The periodicity theory of pitch fares better on two counts than theories that identify colors with objective physical properties.

First, consider the phenomenological distinction betweenunique andbinary hues. Some colors appear to incorporate other colors, and some do not. Purple, for instance, appears both reddish and bluish; red just looks red. Some philosophers contend that the leading physical theories of color cannot explain the unique-binary distinction without essentially invoking the color experiences of subjects. How, for instance, do reflectance classes identified with unique hues differ from those associated with binary hues?

Consider an analogous issue for pitch. Some tones with pitch soundsimple, while other pitched tones,such as sounds of musical instruments, auditorily appear to becomplex and to have discernible components. However, the difference between audibly simple and audibly complex pitched tonesis captured by the simplicity or complexity of a sound's partials. Simple tones are sinusoids, and complex tones have multipleovertones. So, one response is to hold that the unique-binary color distinction and the simple-complex pitch distinction are disanalogous. Unlike the case of color, one might contend, no pitch that is essentially a mixture of other pitches occupies a unique place in pitch space.

Second, considermetamerism. Some surfaces with very differentreflectance characteristics match in color. Metameric pairs share no obvious objective physical property. Some philosophers argue that unless color experience fails to distinguish distinct colors, metamers preclude identifying colors with natural physical propertiesof surfaces (see the entry oncolor).

Now consider the case of pitch. Are there pitch metamers? Some soundswith very different spectral frequency profiles match in pitch. A simple sinusoidal tone at a given frequency matches the pitch of eachcomplex tone with that fundamental frequency (even those that lack a constituent at the fundamental). But, again, the case of pitch differs from the case of color. For each matching pitch, a single natural property does unify the class. The tones all share a fundamental frequency.

3.2.5 Analogies with Color

Two kinds of argument are equally pressing in the case of pitch.

First, arguments fromintersubjective variation transpose. Variations in frequency sensitivity exist among perceivers; for instance, subjects differ in which frequency they identify as middle C. If there is no principled way to legislate whose experience is veridical, pitch might be subjective or perceiver-relative. One response is that, in contrast to the case of unique red, there is an objective standard for middle C: fundamental frequency. But, whose pitch experience has the normative significance to settle the frequency of middle C?

Some might wonder whether there is a pitch analog of the trouble posed by the kind of variation associated withspectrum inversion in the case of color (see the entry oninverted qualia). Spectral shift in pitch, sometimes dramatic, commonly occurs after cochlear implant surgery. This is not spectral inversion for pitch; but, a dramatic shift makes most of the same trouble as inversion. Not quite all the trouble, since cochlear implants preserve the pitchordering and its direction. But, there could be a cochlear implant that switched the placement of electrodes sensitive to 100 hertz and 1000 hertz, respectively; and there could be one that reversed the entire electrode ordering. This goes some distance to grounding the conceivability of a pitch inversion that reverses the height ordering of tones.

Second, consider an argument that frequencies cannot capture therelational structure among the pitches. This is loosely analogous to the argument that physicalism about color fails to capture the relational structure of the hues—for instance, thatred is more similar to orange than either is to green. In the case ofpitch, psychoacoustics experiments show that perceived pitch does not map straightforwardly onto frequency. Though each unique pitch corresponds to a unique frequency (or small frequency range), therelations among apparent pitches do not match those among frequencies. In particular, equivalent pitch intervals do not correspond to equal frequency intervals. For example, the effect upon perceived pitch of a 100 hertz change in frequency varies dramatically across the frequency range. It is dramatic at low frequency and barely detectable at high frequency. Similarly, doubling frequency does not make for equivalent pitch intervals. A 1000 hertz tone must be tripled in frequency to produce the same increase in pitch as that produced by quadrupling the frequency of a 2000 hertz tone. Apparent pitch is a complex function of frequency; it is neither linear nor logarithmic (see, e.g., Hartmann 1997, ch 12, Gelfand 2004, ch 12, Zwicker and Fastl 2006, ch 5). Pitch scales that capture the psychoacoustic data assign equal magnitudes, commonly measured in units calledmels, to equal pitch intervals. Themel scale of pitch thus is an extensive or numerical pitch scale, in contrast to the intensive frequency scale for pitch. The former, but not the latter, preserves ratios among pitches.

S. S. Stevens famously argued on the basis of results drawn from psychoacoustic experiments that pitch is not frequency (see, e.g., Stevens et al. 1937, Stevens and Volkmann 1940). In light of similar results, contemporary psychoacoustics researchers commonly reject theidentification of pitch with frequency or periodicity. The received scientific view thus holds that pitch is a subjective or psychological quality that is no more than correlated with objective frequency (see, e.g., Gelfand 2004, Houtsma 1995). Pitch, on this understanding, belongs only to experiences. The received view of pitch therefore implies an error theory according to which pitch experience involves a widespread projective illusion.

What is the argument against the periodicity theory of pitch? Comparean argument against reflectance physicalism about color. Reflectance physicalism identifies each hue with a class of reflectances. Periodicity physicalism identifies each pitch with a fundamental frequency. In both cases, each determinate sensible feature is identified with a determinate physical property. In the color case, it is objected that reflectance classes do not bear the relations to each other that the colors bear. In the pitch case, the frequencies do not bear the relations to each other that the pitches bear. Thus, if the relational features among a class of sensible qualities are essential to them, an account that does not accurately capture those relations fails. Frequencies, according to this line of argument, do not stand in the relations essential to pitch.

This, of course, is a quite general phenomenon among sensible qualities. Brightness and loudness vary logarithmically with simple physical quantities. Even if we identified candidate molecules for smells, nothing suggests physical similarities would mirror their olfactory similarities.

One might respond, in the case of pitch and other sensible featuresthat can be put in a linear ordering, that the relational order isessential, while the magnitudes are not. In that case, if pitch isfrequency, pitch experience has the right structure, but distortsmagnitudes of difference in pitch. This retains the periodicity theoryand explains away the results in terms of pitch experiences. (Pautz2014, 3.5, has replied that this partial error account cannot bereconciled with certain types of possible intersubjectivedifference).

Suppose, however, we accept that the mel scale is well-founded and that it accurately captures essential relationships among pitches. This does not by itself imply a projective or subjective theory of pitch. Pitches might be dispositions to produce certain kinds of experiences, or they might be simple or primitive properties. It alsois open to seek a more adequate physical candidate for pitch. For instance, pitches might be far more complex physical properties than frequencies. Such physical properties may be of no interest in developing the simplest, most complete natural physical theory, but they may be anthropocentrically interesting.

It is an important question whether a physical theory of sensiblefeatures should just provide a physical candidate for each determinatesensible feature, or whether the physical relationships among thosephysical candidates should capture the structural relations amongsensible qualities (and, if so, which structural relations it shouldcapture). This is an example of how considering in detail the natureand the experience of sensible qualities other than color promisesinsights into traditional debates concerning the sensible qualities(Pautz 2014 contains an empirically-grounded argument concerning avariety of sensible qualities that advances this discussion).

4. Varieties of Auditory Perception

4.1 Musical Listening

Musical listening is a topic that bears on questions about the relationship between hearing sounds and hearing sources. While the philosophy of music has its own vast literature (see the entry onthe philosophy of music), musical experience has not been explored extensively in connection with general philosophical questions about auditory perception. This section discusses links that should advance philosophical work on auditory perception.

4.1.1 Acousmatic Experience

An account of listening to pure or non-vocal music should capture theaesthetic significance of musical listening. Appreciating music is appreciating sounds and sequences, arrangements, or structures of sounds. Thus, the temporal aspects of auditory experiences are critical to appreciatively listening to music.

One might go further and hold that sounds are all that matters inmusic. In particular, some have argued that appreciatively listeningto music demands listening in a way that abstracts from theenvironmental significance, and thus from the specific sources, of thesounds it includes (Scruton 1997, 2–3). Suchacousmaticlistening involves experiencing sounds in a way that is“detached from the circumstances of their production,”rather than “as having a certain worldly cause” (Hamilton2007, 58; see also Hamilton 2009). Listening to music and beingreceptive to its aesthetically relevant features requires notlistening to violins, horns, or brushes on snare drums. It requireshearing sounds and grasping them in a way removed from their commonsources. (Hearing a high fidelity recording thus furnishes anaesthetically identical musical experience despite having a speakercone rather than a violin as source.) “The acousmatic experienceof sound is precisely what is exploited by the art of music”(Scruton 1997, 3).

This suggests an intuitive difference between music and visual artssuch as painting and sculpture. As Kivy (1991) explains, it isdifficult even with the most abstract paintings and sculptures to seethem in a way that takes them to be entirely formal or abstract. Thatis, it is difficult to avoid seeing pictures and sculptures asrepresentational. In contrast, it seems easier to listen attentivelyto the formal acoustical features of musical sounds, without beingcompelled to think of what makes them.

Musical listening thus may be thought to provide a prima facieargument against the claim that in hearing sounds one typically hearssound sources such as the strumming of guitars and bowing ofviolins. If such “interested” audition were the rule,musical listening would be far more challenging.

4.1.2 Acousmatic Listening as Attention to Sounds

Acousmatic experience, however, may be a matter ofattention. Nothing prevents focusing one's attention on the sounds and audible qualities without attending to the instruments, acts, and events thatare their sources, even if each is auditorily available. That musicallistening requires effort and training supports the idea that one candirect attention differently in auditory experience, depending on one's interests. Not getting eaten and safely crossing the street require attending to sound sources, while listening with aesthetic appreciation to a symphony may require abstracting from the circumstances of its production, such as the finger movements of the oboist. This response holds that musical listening is a matter of auditorily attending in a certain way. It is attending to features ofsounds themselves, but does not imply failing to hear sound sources.

The acousmatic thesis is a limited view about which aspects of one's auditory experience are aesthetically significant. These include aspects involved in hearing sounds proper, but exclude, for example, other contents of auditory experience. However, room exists for debate overthe aesthetically significant aspects of what you hear (see Hamilton 2007, 2009). For example, one might argue that live performances have aesthetic advantages over recordings because one hears theperformance of the sounds and songs, rather than their reproduction by loudspeakers (cf. Mag Uidhir 2007). Circumstances of sound production, such as that skillful gestures generate a certain passage, or that a particularly rare wood accounts for a violin's sounds, might be aesthetically relevant in a way that outstrips the sounds, and some such features may be audible in addition to sounds. For instance, hearing the spatial characteristics of a performance may hold aesthetic significance beyond the tones and structures admitted by traditional accounts of musical listening. Composers may even intend “spatial gestures” among aspects essential for the appreciation of a piece (see, e.g., Solomon2007). To imagine auditorily experiencing the spatial characteristicsof music in a way entirely divorced from the environmental significance of the sounds is difficult. Appreciating the relationship between experiences of sounds and of sources makes room for a view of the aesthetic value of musical listening that is more liberal than acousmatic experience allows.

4.2 Speech Perception

4.2.1 Is Speech Special?

Speech perception presents uniquely difficult twists, and fewphilosophers have confronted it directly (Appelbaum 1999, Trout 2001a,Matthen 2005, ch 9, and Remez and Trout 2009 are recentexceptions). Something striking and qualitativelydistinctive—perhaps uniquely human—seems to set theperception of speech apart from ordinary hearing. The mainphilosophical issues about speech perception concern versions of thequestion, Is speech special? (See O'Callaghan 2014 for acomprehensive review and discussion.)

How does perceiving speech differ from perceiving ordinarynon-linguistic sounds? Listening to music and listening to speech eachdiffer from listening to other environmental sounds in the followingrespect. In each case, one's interest in listening is to some degreedistanced from the specific environmental happenings involved in theproduction of sounds.

But this is true of listening to music and of listening to speech fordifferent reasons. In music, it is plausible that one's interest isin the sounds themselves, rather than in the sources of theirproduction. However, speech is a vehicle for conventional linguisticmeaning. In listening to speech, one's main interest is the meanings,rather than in the sources of sound. Ultimately, the informationconveyed is what matters.

Nevertheless, according to the most common philosophicalunderstanding,perceiving spoken utterances is just a matter ofhearing sounds. The sounds of speech are complex audible soundstructures. Listening to speech in a language you know typicallyinvolvesgrasping meanings, but grasping meanings requiresfirst hearing the sounds of speech. According to this account,grasping meanings itself is a matter of extra-perceptualcognition.

The commonplace view—that perceiving speech is a variety ofordinary auditory perception that just involves hearing the sounds ofspeech—has been challenged in a number of ways. The challengesdiffer in respect of how speech perception is held to differ fromnon-linguistic audition.

4.2.2 The Objects of Speech Perception

First, consider theobjects of speech perception. What are theobjects of speech perception, and do they differ from those ofordinary auditory perception? According to the commonplaceunderstanding, hearing speech involves hearing sounds. Thus, hearingspoken language shares perceptual objects with ordinary audition.Alternatively, one might hold that the objects of speech perceptionare not ordinary sounds at all. Perhaps they are language-specificentities, such asphonemes orwords. Perhaps, as somehave argued, perceiving speech involves perceivingarticulatorygestures or movements of the mouth and vocal organs (see thesupplement onSpeech Perception: Empirical and Theoretical Considerations). Note that if audition's objectstypically include distal events, speech in this respect is notspecial, since its objects do not belong to an entirely different kindfrom ordinary sounds.

4.2.3 The Contents of Speech Perception

Second, consider thecontents of speech perception. Does the content of speech perception differ from that of ordinary audition? If it does, how does the experience of perceiving speech differ from that of hearing ordinary sounds? Perceiving speech might involve hearing ordinary sounds but auditorily ascribing distinctivefeatures to them. These features might simply be, or comprise,finer grained qualitative and temporal acoustical details than non-linguistic sounds audibly possess. But perceiving speech also might involve perceiving sounds as belonging tolanguage-specific types, such as phonemes, words, or other syntactic categories.

Furthermore, speech perception's contents might differ in a moredramatic way from those of non-linguistic audition. Listening withunderstanding to speech involves graspingmeanings. Thecommonplace view is conservative. It holds that grasping meanings isan act of theunderstanding rather than of audition. Thus, thedifference between the experience of listening to speech in a languageyou know and the experience of listening to speech in a language youdo not know is entirelycognitive.

But one might think that there also is aperceptualdifference. A liberal account of this perceptual difference holds thatperceiving speech in a language you know may involve hearing soundsas meaningful or auditorily representing them as havingsemantic properties (see, e.g., Siegel 2006, Bayne 2009; see alsoAzzouni 2013). Alternatively, a moderately liberal account holds thatthe perceptual experience of speech in a language you know involvesperceptually experiencing language-specific but neverthelessnon-semantic features (see O'Callaghan 2011b).

4.2.4 Is Speech Perception Auditory?

Third, consider theprocesses responsible for speechperception. To what extent does perceiving speech implicate processesthat are continuous with those of ordinary or general audition, and towhat extent does perceiving speech involve separate, distinctive, ormodular processes? While some defendgeneral auditory accountsof speech perception (see, e.g, Holt and Lotto 2008), some argue thatperceiving speech involves dedicated perceptual resources, or even anencapsulated perceptual system distinct from ordinary non-linguisticaudition (see, e.g., Fodor 1983, Pinker 1994, Liberman 1996, Trout2001b). These arguments typically are grounded in several types ofphenomena, including themulti modality of speechperception—visual cues about the movements of the mouth andtongue impact the experience of speech, as demonstrated by the McGurkeffect (see the section4.3 Crossmodal Influences);duplex perception—a particular stimulussometimes contributes simultaneously both to the experience of anordinary sound and to that of a speech sound (Rand 1974); and thetop-down influence of linguistic knowledge upon the experienceof speech. A reasonable challenge is that each of thesecharacteristics—multimodality, duplex perception, and top-downinfluence—also is displayed in general audition.

See the supplement onSpeech Perception: Empirical and Theoretical Considerations.

4.3 Crossmodal Influences

4.3.1 Crossmodal Illusions

Auditory perception of speech is influenced by cues from vision and touch (see Gick et al. 2008). The McGurk effect in speech perception leads to an illusory auditory experience caused by a visual stimulus (McGurk and Macdonald 1976). Do such multimodal effects occur in ordinary audition? Visual and tactile cues commonly do shape auditoryexperience. The ventriloquist illusion is an illusory auditory experience of location that is produced by an apparent visible sound source (see, e.g., Bertelson 1999). Audition even impacts experience in other modalities. The sound-induced flash illusion involves a visual illusion as of seeing two consecutive flashes that is produced when a single visible flash is accompanied by two consecutive audible beeps (Shams et al. 2000, 2002). Such crossmodal illusions demonstrate that auditory experience is impactedby other modalities and that audition influences other modalities. Ingeneral, experiences associated with one perceptual modality are influenced by stimulation associated with other modalities.

4.3.2 Causal or Constitutive?

An important question is whether the impact is merely causal, or whether perception in one modality is somehow constitutively tied to other modalities. If, for instance, vision merely causally impacts your auditory experience of a given sound, then processes associated with audition might be proprietary and characterizable in terms that do not appeal to other modalities.

Suppose that such cross-modal illusions are intelligible as the results of adaptive perceptual strategies. In ordinary circumstances, crossmodal processes serve to reduce or resolve apparent conflicts ininformation drawn from several senses, and thereby make perception more reliable overall. Thus, crossmodal illusions differ fromsynaesthesia. Synaesthesia is just a kind of accident. It results from mere quirks of processing, and it always involves illusion (or else is accidentally veridical). Crossmodal recalibrations, in contrast, are best understood as attempts “to maintain a perceptual experience consonant with a unitary event” (Welch and Warren 1980, 638).

This principled reconciliation of information drawn from differentsensory sources suggests, first, that audition is governed byextra-auditory perceptual constraints. Second, since conflict requiresa common subject matter, such constraints concern the common sourcesof stimulation to multiple senses. This suggest that audition andvision share a perceptual concern for a common subject matter.

This concern for a common subject matter may be reflected in certainforms of perceptual experience. For instance, the commonality may beexperientially evident in the experience of space and the perceptualexperience of audio-visual intermodal feature binding. Thus, auditoryand visual experiences may share a common multimodal or amodal contentor character (see O'Callaghan 2008b, Clark 2011). And one modalitycould have a constitutive rather than a merely causal impact uponprocesses and experiences associated with another.

4.3.3 Multimodality in Perception

What hangs on this? First, it bears on questions about audition's content. If we cannot exhaustively characterize auditory experience in terms that are modality-specific or distinctive to audition, and doing so requires amodal or multimodal contents, then we might hearas of things we can see or experience with other senses. This is related to one puzzling question about hearing sound sources: How could you hear as of something you could see? Rather than just a claim about audition's content that requires further explanation, we now have a story about why things like sound sources figure in the content of auditory experience. Second, all of this may bear on how to delineate what counts as auditory perception, as opposed to visualor even amodal perception. If hearing is systematically impacted by visual processes, and if it shares content and phenomenology with other sense experiences, what are the boundaries of auditory perception? Multimodal perception may bear on the question of whetherthere are clear and significant distinctions among the sense modalities (cf. Nudds 2003). Finally, multimodal perceptual experiences, illusions, and explanatory strategies may illuminate thephenomenologicalunity of experiences in different modalities,or the sense in which, for instance, an auditory experience and a visual experience of some happening comprise a single encompassing experience (see the entry onthe unity of consciousness).

We can ask questions about the relationships among modalities indifferent areas of explanatory concern. Worthwhile areas for attentioninclude the objects, contents, and phenomenology of perception, aswell as perceptual processes and their architecture. Crossmodal andmultimodal considerations might shed doubt on whether vision-basedtheorizing alone can deliver a complete understanding of perceptionand its contents. This approach constitutes an importantmethodological advance in the philosophical study of perception (seeO'Callaghan 2012 for further discussion).

5. Conclusion and Future Directions

Considering modalities other than vision enhances our understanding ofperception. It is necessary to developing and vetting an adequatecomprehensive and general account of perception and its roles.Auditory perception reveals a new rich territory for philosophicalexploration in its own right, but it also provides a useful contrastcase in which to evaluate claims about perception proposed in thevisual context. One of the most promising directions for future workconcerns the nature of the relationships among perceptual modalities,how these relationships shape experience across modalities, and howthey may prove essential to understanding perception itself. Recentphilosophical work on auditory perception thus encourages the advancebeyond considering modalities in isolation from each other.

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color |consciousness: unity of |music, philosophy of |perception: the contents of |perception: the problem of |qualia: inverted |sounds

Acknowledgments

I am very grateful to David Chalmers, Maddy Kilbride, and ShaunNichols for extensive and helpful comments on drafts of a previousversion of this entry.