## 1. Other Modalities and the Philosophy of Perception



The philosophy of sounds and auditory perception is one area of 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 is worthwhile to discuss
the motivation and rationale for this kind of work.



Philosophical thinking about perception has focused predominantly on
vision. The philosophical puzzle of perception and its proposed
solutions have been shaped by a concern for visual experience and
visual illusions. Questions and proposals about the nature of
perceptual content have been framed and evaluated in visual terms, and
detailed accounts of what we perceive frequently address just the
visual case. Vision informs our understanding of perception's
epistemological role and of its role in guiding action. It is not a
great exaggeration to say that much of the philosophy of perception
translates roughly as philosophy of visual perception.



Recently, however, other perceptual modalities have attracted
attention (see, e.g., Stokes et al. 2015, Matthen 2015). In addition
to auditory perception and the experience of sound, touch and tactile
awareness have generated philosophical interest concerning, for
instance, the tactile and proprioceptive experience of space, the
objects of touch, whether contact is required for touch, and whether
distinct modalities detect pressure, heat, and pain (see, e.g.,
O'Shaughnessy 1989, Martin 1993, Scott 2001, Fulkerson 2013,
2016). The unique phenomenology of olfaction and smells has been used
to argue that vision is atypical in supporting the transparency of
perceptual experience (Lycan 2000, 282; cf. Batty 2010) and that
perceptual objectivity does not require spatiality (Smith 2002, ch 5).
Lycan (2000) even suggests that the philosophy of perception would
have taken a different course had it focused upon olfaction instead of
vision (see also Batty 2011). Some authors have appealed to taste and
flavor to challenge traditional ways of dividing and counting senses
(Smith 2015; cf. Richardson 2013).



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



First, one might take work on non-visual modalities as
*translating* what we have learned from the visual case into
terms that apply to other modalities. This approach is relatively
conservative. It assumes that vision is representative or paradigmatic
and that we have a good understanding of perception that is derived
from the case of vision. One example of this kind of approach would be
to develop an account of the representational content of auditory
experience.



Second, considering other modalities might *extend* our
vision-based understanding of perception. Non-visual cases might draw
attention to new kinds of phenomena that are missing from or not
salient in vision. If so, a vision-based account of perception is
satisfactory as far as it goes, but it leaves out critical pieces. For
example, speech perception, multimodal perception, and flavor
perception might involve novel kinds of perceptual phenomena absent
from the visual case.



Third, considering other modalities might *challenge*
vision-based claims about perception. If falsifying evidence is
discovered in non-visual cases, then theorizing beyond vision may
force revision of general claims about perception that are supported
by vision. For example, if olfactory experience is not diaphanous, but
olfactory experience is perceptual, the transparency thesis for
perceptual experience fails.



Finally, we might attempt to determine whether any *unified*
account exists that applies generally to all of the perceptual
modalities. We can ask this question either at the level of quite
specific claims, such as those concerning the objects of perception or
the nature and structure of content. We can ask it about the
relationships among perceiving, believing, and acting. Or we can ask
it about the general theory of necessary and sufficient conditions for
perceiving. Some philosophers, impressed by findings concerning
non-visual modalities, express skepticism whether a unified theory
exists (e.g., Martin 1992).



Whatever the approach, extending our knowledge about perception beyond
the visual requires systematic attention to individual modalities as
well as careful accounting in order to determine how the results bear
on general questions about perception. Whatever the outcome, audition
is a rich subject matter in its own right, and investigating this
subject matter is crucial to our overall understanding of
perception.


## 2. The Objects of Auditory Perception



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


### 2.1 Sounds



In the first instance, typical human perceivers hear *sounds*. It
is plausible that sounds are objects of auditory perception.



What are sounds? Sounds traditionally have been counted with colors,
smells, and tastes as *secondary*, *sensible*, or
*sensory* *qualities* (see, e.g., Locke 1689/1975, Pasnau
1999, 2000, Leddington 2019). However, recently it has been proposed
that sounds are *individuals* to which sensible features are
attributed. In particular, several philosophers have proposed that
sounds are public, distally-located, event-like individuals (Casati
and Dokic 1994, 2005, O'Callaghan 2007, Matthen 2010).



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


#### 2.1.1 Private or Public?



Are sounds private or public? Maclachlan (1989) argues that the sounds
we hear are *sensations* (rather than, for instance, the pressure
waves that cause auditory experiences). Such sensations are internal
and private, and we experience them directly, or without apparent
mediation. On Maclachlan's account, we hear the ordinary things
and happenings that are the *sources* of sounds only
*indirectly*, by means of inference from auditory data.



Maclachlan's story is noteworthy partly because he uses hearing
and sounds to motivate a general claim about perception. He claims
that what seems perfectly intuitive and obvious in the case of sounds
and hearing--that something other than material objects are the
direct objects of hearing; that the direct objects of audition are
internal; and that we indirectly hear things in the world by hearing
their sounds--helps us to discover what is true of all
perception. According to Maclachlan, for instance, seeing involves
direct awareness of sensations of patterns of light, while surfaces
and ordinary objects figure only indirectly and thanks to inference
among the intentional objects of sight. The case of sounds and
audition is important because it reveals that perceiving involves
awareness of sensations in the first instance, and of the external
world only indirectly.



Maclachlan's description of sounds and auditory experience has
some attractions. First, sounds are among the things we hear. And
sounds are among the direct or immediate objects of audition in the
relatively innocuous sense that hearing a sound does not seem to
require hearing as of something else. Hearing a collision, on the
other hand, may seem to require awareness as of a sound. Furthermore,
sounds are unlike the ordinary material objects (e.g., bottles and
staplers) we see. You cannot reach out and grab a sound, or determine
its temperature. Instead, sounds may strike us as byproducts or
effects of such ordinary things and their transactions. Sounds result
from activities or interactions of material bodies and thus are
experienced as distinct or independent from them (cf. Nudds 2001).
Nevertheless, audition does afford some variety of awareness of the
sources of sounds, or at least provides information about them.



However, the claim that sounds are sensations is unattractive. Good
reasons suggest that sounds are public rather than private, even if
sounds are not identical with ordinary objects and events such as
clothespins and collisions. Suppose I am near the stage in a hall
listening to some music, and that I have a headache. It is a confusion
to think you could feel my headache, but I assume you hear the sounds
I hear. Suppose I move to the back of the hall, and the headache then
gets better. My experience of the sounds of the music differs once I
am at the back of the room, and my experience of the headache differs.
The sound of the music itself need not differ (the musicians could
make the same sounds), but the headache itself changes. The sounds can
continue once I leave the room, but if I stop experiencing the
headache, it is gone. Moreover, the notion of an unfelt headache is
puzzling, but it makes good sense to say that a tree makes a sound
when it falls in the woods without being heard. Finally, tinnitus, or
ringing of the ears, is an illusory or hallucinatory experience as of
a sound, but received wisdom maintains that there are no illusory
headaches.



This suggests that audition does not provide *special* reasons to
believe that the objects of perception are private sensations. Sounds,
construed as objects of auditory perception, plausibly inhabit the
public world. (See the section
 3.1 Spatial Hearing
 for further discussion.)


#### 2.1.2 Proximal or Distal?



Are sounds proximal or distal? The customary science-based view holds
that sounds are pressure waves that travel through a medium (see also
Sorensen 2008). On this account, sounds are caused by objects and
events such as collisions, and sounds cause auditory experiences.
However, sounds are not auditorily experienced to travel through the
surrounding medium as waves do. Thus, if sounds are waves, then the
sounds we hear may be proximal, located at the ear of the hearer.



Alternatively, some have argued that audition presents sounds as being
located in some direction at a distance (Pasnau 1999,
O'Callaghan 2007, ch. 3, 2010). On such an account, sounds
commonly appear auditorily to be in the neighborhood of their sources
and thereby furnish useful information about the locations of those
sources. The sound of the drumming across the street seems to come
from across the street but does not seem audibly to travel. When
sounds do appear to fill a room, sound seems located all around.
Sounds that seem to "bounce" around a room appear
intermittently at different locations rather than as traveling
continuously from place to place. Experiencing a missile-like sound
speeding towards your ears illustrates the contrast with ordinary
hearing (O'Callaghan 2007, 35). Sounds, according to this
conception, ordinarily appear to have distal locations and to remain
stationary relative to their sources.



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



One might object by resisting the phenomenological claim that we
experience sounds as distally located, for instance by suggesting that
audition is aspatial, or that audition is spatial but sound
*sources* rather than sounds are auditorily localized (see
section
 3.1 Spatial Hearing
 for further discussion). Or, one might accept some measure of
illusion. Another possibility is that we experience only a small
subset of the locations sounds occupy during their lifetimes (for
instance, while at their sources), and simply fail to experience where
they are at other times. This avoids ascribing illusion. Finally,
Fowler (2013) argues indirectly on the basis of echoes against distal
theories 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 whistles
*have* or *possess* rather than *make* or
*produce* 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 transience in 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 stable disposition). This account
implies that sounds sometimes make sounds they do not have, and that
they have sounds when silent. One also might ask whether events such
as collisions and strummings, rather than objects, bear sounds.
Leddington (2019) recently has defended such an account.



A revisionist challenge comes from those who argue that sounds are
*individuals* rather than properties. Several arguments support
this understanding. First, empirical work on *auditory scene
analysis* suggests that one primary task of audition is to carve up
the acoustic scene into distinct sounds, each of which may possess its
own pitch, timbre, and loudness (Bregman 1990). Multiple distinct
sounds 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 audible
features 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 sensible
qualities.



Several responses to these arguments are available (see Cohen 2009 for
the most developed reply). 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 that survive 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. However, 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 section
 3.2 Audible Qualities).
 


#### 2.1.4 Objects or Events?



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



No contemporary philosopher has yet claimed that sounds are objects
*in the ordinary sense*. Those who argue that sounds are
individuals commonly point out that sounds not only persist and
survive change (as do ordinary material objects), but also
*require* time to occur or unfold. It is difficult to imagine an
instantaneous sound, or one that lacks duration. Sounds are not
commonly treated as existing wholly at a given moment during their
duration. Indeed, the identities of many common sounds are tied to
patterns of change in qualities through time. The sound of an
ambulance siren differs from that of a police siren precisely because
the two differ in patterns of qualitative change through time. The
sound of the spoken word 'team' differs from that of
'meat' because each instantiates a common set of audible
qualities in a different temporal pattern. These considerations
support the view that sounds are event-like individuals (see Casati
and Dokic 1994, 2005, Scruton 1997, O'Callaghan 2007, Matthen
2010).



This may bear on debates about persistence in the following way.
Differences in the intuitive plausibility of endurantism and
perdurantism may be grounded in facts about perception. In particular,
vision may treat objects as persisting by enduring or being wholly
present at each time at which they exist, while audition may treat its
objects as persisting by perduring or having temporal parts. This may
stem from differences in perceptual organization. For instance,
exhibiting a visible property profile at a time may suffice for being
a visual object of a given sort, while being an audible object of a
given sort may require exhibiting an audible property profile over
time.


### 2.2 Auditory Objects



Though most philosophers construe sounds either as properties or as
event-like individuals (see section
 2.1 Sounds),
 psychologists commonly have discussed *auditory objects* (see,
e.g., Kubovy and Van Valkenburg 2001, Griffiths and Warren 2004, Heald
et al. 2017). The target of such discussion is not simply
audition's *intentional* objects or *proper* (specific
to audition) objects. The intended analogy is with *visual
objects*. Talk of auditory objects gestures at the visual processes
involved in perceptually discriminating, attentively tracking,
recognizing and categorizing ordinary material objects. What justifies
talk of *object perception* in audition?


#### 2.2.1 Object Perception in Audition



First of all, humans typically do not auditorily perceive
three-dimensional, bounded material objects *as such*, though it
is plausible to think we visually perceive them. Hearing does not
resolve the edges, boundaries, and filled volumes in space that I see,
and I do not hear audible items to complete spatially behind occluders
as do visible surfaces of objects. If perceiving a three-dimensional
object requires awareness of its edges, boundaries, and extension,
perhaps in order to discriminate it from its surroundings, humans
typically 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 in a
more general sense that is common to both vision and audition
(O'Callaghan 2008a).



Perceiving objects requires parsing a perceptual scene into distinct
*units* that one can attend to and distinguish from each other
and from a background. In vision, bounded, cohesive collections of
surfaces that are extended in space and that persist through time play
this role (see, e.g., Spelke 1990, Nakayama et al. 1995, Leslie et al.
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 of auditory scene
analysis in section
 2.1 Sounds).
 A critical difference, however, is that while vision's objects
are extended in space, and are individuated and recognized primarily
in virtue of spatial characteristics, audible individuals are extended
in time, and are perceptually individuated and recognized primarily in
virtue of pitch and temporal characteristics (see, e.g., Bregman 1990,
Kubovy and Van Valkenburg 2001). For instance, audible individuals
have temporal edges and boundaries, and boundary elements can belong
only to a single audible individual. They also are susceptible to
figure-ground effects over time. One can, for instance, shift
attention among continuous audible individuals that differ in pitch.
Furthermore, they are susceptible to completion effects over time in
much the same way that visible objects are perceptually completed in
space. Seeing a single visible region to continue behind a barrier is
analogous to hearing a sound stream to continue through masking noise,
which may take place even when there is no corresponding signal
(Bregman 1990, 28). Finally, multiple distinct, discrete audible
individuals, such as the temporally bounded notes in a tune, can form
audible *streams* that comprise a single perceptible unit. Such
streams are subject to figure-ground shifts, and, like collections of
surfaces, they can be attentively tracked through changes to their
features and to one's perspective. Though such complex audible
individuals include sounds, they comprise temporally unified
collections of sounds and silence that are analogous to spatially
complex visible objects, such as tractors.



Such audible individuals are temporally extended and bounded, serve as
the locus for auditory attention, prompt completion effects, and are
subject to figure-ground distinctions in pitch space. For these
reasons, the auditory processes involved in their perception parallel
those involved in the visual perception of ordinary three-dimensional
objects. The parallels suggest a shared sense in which vision and
audition involve a more general form of object perception (see, e.g.,
Kubovy and Van Valkenburg 2001, Scholl 2001, Griffiths and Warren
2004, O'Callaghan 2008a, Matthen 2010).


#### 2.2.2 What is an Auditory Object?



What is the shared sense in which both visible and audible individuals
deserve to be called 'objects'? Kubovy and Van Valkenburg
(2001, 2003) define objecthood in terms of figure-ground segregation,
which requires perceptual grouping. They propose the *theory of
indispensable attributes* as an account of the necessary conditions
on perceptual grouping (see also Kubovy 1981). Indispensable
attributes for a modality are those without which perceptual
numerosity is impossible. They claim that while space and time are
indispensable attributes for vision (and color is not), pitch and time
are indispensable attributes for auditory objects. Though they are
more skeptical about whether audition parallels vision, Griffiths and
Warren (2004) sympathize with a figure-ground characterization but
suggest a working notion of an auditory object defined in terms of
"an acoustic experience that produces a two-dimensional image
with 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, which
facilitates 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, time
plays 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 and
perception is relatively new among philosophers (see, e.g.,
O'Callaghan 2008a, and essays in Bullot and Egre 2010,
including Matthen 2010, Nudds 2010). Such work has led to the
development of general accounts of perceptual objects designed to
avoid visuocentrism (see, e.g., O'Callaghan 2016, Green 2019).
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 hear
anything else? Reflection *suggests* we hear things beyond sounds
and sound complexes. In hearing sounds, one may seem to experience the
backfiring of the car or the banging of the drum. One might hold that
a primary part of audition's function is to reveal *sound
sources*, 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, as
Leddington (2019) holds, then hearing a tuba or the playing of a tuba
might only require hearing its sounds. Perceptually ascribing such
audible attributes to their sources might ground epistemically
unmediated awareness of tubas or their playings.



However, the individuals to which audible attributes are perceptually
attributed need not be identical with ordinary objects or events.
Instead, audible attributes may belong in the first instance to
sounds. Sounds plausibly are distinct from ordinary or extra-acoustic
individuals (O'Callaghan 2007, 2011). Suppose then that one
could not hear an ordinary object or event without there existing an
audible 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 happenings
connected with sounds might seem to require inference, association, or
some otherwise cognitive process, and so awareness of a sound source
might appear to always involve more than perceptual awareness.
According to such an account, awareness of environmental things and
happenings thanks to audition is epistemically mediated by awareness
as of sounds and auditory objects, but does not itself constitute
auditory perceptual awareness as of those things and happenings. You
are inclined to think you hear the source because your representing or
being aware of it co-occurs with, but is no more than a downstream
consequence triggered by, your auditory experience.



Such an account is not wholly satisfactory. First, the phenomenology
of audition suggests something stronger than indirect, epistemically
mediated awareness of things such as collisions or guitar strummings
or lions roaring. Reflection suggests auditory awareness *as of*
collisions, strummings, and lions. Second, the capacity to refer
demonstratively to such things and events on auditory grounds also
suggests genuine perceptual awareness of them. Third, we commonly
perceptually 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 a
dripping faucet." We distinguish two quite similar rattles once
we hear one as of a muffler clamp and the other as of a loose fender.
Furthermore, characterizing certain audible features and explaining
perceptual constancy effects involving such features requires appeal
to sound sources. Handel says of timbre: "At this point, no
known acoustic invariants can be said to underlie timbre... The cues
that determine timbre quality are interdependent because all are
determined by the method of sound production and the physical
construction of the instrument" (Handel 1995, 441). Explaining
loudness constancy--why moving to the back of the room does not
change how loudly the lecturer seems to speak--appeals to facts
about the sources of sounds (Zahorik and Wightman 2001). Auditory
processing proceeds in accordance with natural constraints concerning
characteristics of sound sources, and information concerning sources
shapes how auditory experiences are organized. This is to say that
processes responsible for auditory experience proceed as if acoustic
information is information about sound sources. Finally,
audition-guided action supports the claim that we hear such things and
events. Turning to look toward the source of a sound or ducking out of
the way of something we hear to be approaching--behaviors guided
by auditory experience--would make little sense if we heard only
sounds. In the first place, these reasons ground a case for thinking
that auditory perceptual experience does not strictly end with sounds
and auditory objects. In particular, awareness as of a source, even if
dependent upon awareness as of a sound, may be constitutive of
one's auditory perceptual experience.



The main barrier to an alternative is that the relation between sounds
and ordinary things or happenings is commonly understood as
*causal* (see, e.g., Nudds 2001). Awareness as of an effect does
not itself typically furnish epistemically unmediated awareness of its
cause. Seeing smoke is not seeing fire. The right sort of dependence
between characteristics of the experience and the cause is not
apparent, and awareness as of an effect does not by itself ground
perceptual demonstratives that concern the cause. The metaphysical
indirectness of the causal relation appears to block epistemic
directness (see O'Callaghan 2011a for further discussion).


#### 2.3.2 The Mereology of Sounds and Sources



Is there another explanatory route? Suppose that instead of a causal
relation, we understand the relationship between sounds and sources
*mereologically*, or as one of part to whole (see
O'Callaghan 2011a). Parthood frequently does ground perceptual
awareness. For instance, seeing distinct parts of a surface
interrupted by an occluder leads to perceptual experience as of a
single surface (imagine seeing a dog behind a picket fence). Seeing
the facing surfaces of a cube affords awareness as of a cube, and we
can attentively track that same cube as it rotates and reveals
different surfaces. Suppose, then, that a sound is an event-like
individual (recall, property accounts escape the worry). This event is
part of a more encompassing event, such as a collision or the playing
of a trumpet, that occurs in the environment and that includes the
sound. So, the typical horse race includes the sounds, and you might
auditorily perceive the racing in hearing some of its proper parts:
the sounds. More specifically, you may hear the galloping thanks to
hearing the sounds it includes. You may fail to hear certain parts of
the racing event, such as the jockey's glance back after
crossing the wire, but you also fail to see parts of the race, such as
the misstep of the horse in second place. If the sounds are akin to
the audible "profile" of the event, analogous to the
visible surfaces of objects and visible parts of events, you might
then enjoy auditory awareness as of the galloping of the horses in
virtue of your awareness as of the sounds of the hooves. The sound is
not identical with the galloping, and it is not just a property or a
causal byproduct of the galloping. It is a part of a particular event
of galloping. The metaphysical relation of part to whole, in contrast
to that between effect and cause, might ground the sort of
epistemically unmediated awareness of interest (cf. Nakayama et al.
1995, Bermudez 2000, Noe 2004). Auditory perceptual
awareness as of the whole may occur thanks to experiencing the
part.



One objection is that this mereological account of the relation
between sounds and sources cannot account for hearing ordinary
*objects* by hearing their sounds. You could not strictly hear a
tuba by hearing its sound because a tuba is not an event of which a
sound is a part. However, the sound is part of the event of playing
the tuba, and the tuba is a participant in that playing. So, though
you are not aware as of a tuba, you are aware as of an event that
involves a tuba. That perhaps is enough to explain talk of hearing
tubas and to assuage the worry (for further discussion, see Young
2018).



Another more serious objection contends that the events we seem to
hear are ones that do not constitutively involve sounds or that might
have taken place without sounds. For instance, we hear the collision,
but the collision is something that could have occurred in a vacuum
and not made a sound. (Note that this differs from the claim discussed
below according to which sounds are identical with source events and
so inaudibly exist in vacuums.) If so, the collision and the sound
differ and the collision does not strictly include a sound. The
collision therefore must have made the sound as a causal byproduct.
This suggests that, strictly speaking, you could not hear that very
collision event (since it causes the sound). The best response is to
bite the bullet and accept that events that do occur or that could
occur in vacuums cannot be heard since they include no sounds. This is
not so bad, since you could hear a different, more encompassing event
that includes a sound along with a collision. Alternatively, one might
say the very same event that occurs in a vacuum also could occur in
air, but that it would have involved a sound had it occurred in air.
In that case, one can only hear such events when they occur in air and
include a sound. The choice depends upon one's metaphysics of
events. In either case, it seems reasonable that token events that do
not include sounds are inaudible.



Casati et al. (2013) sidestep some of these concerns by rejecting the
distinction between sounds and events that are typically understood as
sound sources. They propose to "Ockhamize" the
"event sources" of sounds by identifying sounds with
events such as collisions and vibrations. The sound just is the
collision or the vibrating, whether or not it occurs in air. This
account 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. So, whether or not we hear sound sources
impacts the epistemology of audition. 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 affects 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 the *contents* 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.
Parallel questions can be raised without relying on the perceptual
content framework, though important complications arise.


### 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 of things
*as* 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, there is a natural account of being so informed. 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 having those experiences.



But learning about spatial features on the basis of audition and
audition's bearing information about space both are consistent
with entirely aspatial auditory phenomenology. For instance, volume
might bear information about distance, and differences in volume at
the two ears might bear information about direction. In that case,
audition bears information about space, and learning about space on
the basis of audition is possible, but it does not follow that
auditory experience is spatial or that audition represents space.


#### 3.1.1 Skepticism about Spatial Audition



Notably, a tradition of skepticism about audition's spatiality
exists in philosophy. Certainly, our capacity to glean information
about space is less acute in audition than in vision. Vision reveals
fine-grained spatial details that audition cannot convey, such as
patterns and textures. But philosophers who are skeptical about
spatial audition are not just concerned about a difference in spatial
*acuity* 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 of establishing in a
> place', but 'the act of discovering the place of'.
> Even so 'location' is misleading, because it implies that
> there is such a thing as discovering the place of sounds. Since sounds
> do not have places there is no such act. (Malpas 1965, 131)
> 



O'Shaughnessy states, "...We absolutely never
immediately perceive sounds to be *at* any place. (Inference from
auditory data being another thing)" (O'Shaughnessy 2002,
446). The claim is 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
loudness and 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 the
conclusion that hearing sounds involves auditory awareness of
sensations involves denying that audition satisfies spatial
prerequisites on experiencing sounds as objective or public. For
instance, Maclachlan (1989) claims that audition's
phenomenology--in particular, its aspatial
phenomenology--provides reasons to think sounds are sensations.
Comparing sounds with pains, which we readily recognize as sensations,
he says, "[A]lthough the sounds we hear are just as much effects
produced in us as are the pains produced by pins and mosquitoes,
*there is no variety in the location of these effects* [the
sounds]. Because of the lack of contrast, we are not even aware that
the sounds we hear are bodily sensations" (Maclachlan 1989, 31,
my emphasis). Maclachlan means that, in contrast even to the case of
pains, which are felt at different bodily locations, sounds are not
experienced to be at differing locations, and so we are not even
inclined to recognize that they are bodily sensations. Maclachlan then
suggests that we *associate* sounds with things and happenings
outside the body rather than appreciate that they are effects in us.
Given the lack of spatial *variation* among experienced sounds,
we projectively associate sounds with distal sources. This explanation
assumes that experienced sounds exhibit no audibly apparent spatial
variation: sounds seem located at the ears or lack apparent location
altogether. Denying that auditory experiences present sounds at
varying locations beyond the ears invites difficulty in finding a
place for sounds in the world. If audition is wholly aspatial, this
may encourage a retreat to the view that 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 thriving
*empirical* 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 cannot match
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 on
*phenomenological* 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 in many ordinary
cases perceptually experienced as located in the environment at a
distance in some direction. We hear the sound of the knocking over
near the door; we hear footsteps approaching from behind and to the
left; hearing sound to "fill" a room is itself 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 of *Individuals* that because auditory experience is
not *intrinsically* spatial--spatial concepts have no
intrinsic auditory significance--a purely auditory experience
would be non-spatial. Thus, it 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 have certain
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 or
touch, it could mean that audition *depends* 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 other
intrinsically spatial modalities. Second, it might be understood as a
claim about the *objects* 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. However, if
any modality *in isolation* ever could ground spatial experience,
audition could. On one hand, given the mechanisms of spatial hearing,
it is empirically implausible that a normal acoustic environment with
rich spatial cues would fail to produce even a minimally spatial
purely auditory experience. Even listening only to stereo headphones
could produce a directional auditory experience. On the other hand, it
does seem possible that there could be a non-spatial but
*impoverished* exclusively auditory experience if no binaural or
other 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 materials
for 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 internal
*structure* of audition:



> 
> When we see (or seem to see) something, we see it as occupying or as
> located within a region of space; when we hear (or appear to hear) a
> sound we simply hear the sound, and we don't experience it as
> standing in any relation to the space it may in fact occupy. (Nudds
> 2001, 213-14)
> 



Audition, unlike vision, lacks a spatial structure or field, claims
Nudds. A purely auditory experience thus would not comprise a spatial
field 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 ring
*as empty*. In audition, Nudds claims, one never experiences a
space as empty or unoccupied.



In response, one might simply deny a difference between vision and
audition in this respect. If one can attend to a location near the
center of the visible ring as empty, one can attend to the location
between the sounding alarm clock and the slamming door as a place
where there is no audible sound--as acoustically empty space. Of
course, auditory space generally is less replete than visual space,
but this is contingent. Consider seeing just a few stars flickering on
and off against a dark sky. Since such an experience may have spatial
structure, and since it is analogous to audition, one might on these
grounds defend the claim that audition has spatial structure (see also
Young 2017).


#### 3.1.4 How Spatial Audition Differs from Spatial Vision



What about the second way mentioned above to understand
Strawson's claim? Though audition's status as
intrinsically spatial may not differ from that of vision or touch,
perhaps sounds are not intrinsically spatial. But without further
argument, or a commitment to a theory of sounds, it is difficult to
state confidently the intrinsic features of sounds and thus whether
they include spatial features. If, for instance, wavelength is among a
sound's intrinsic features, sounds are intrinsically
spatial.



Nonetheless, the claim might be that, as they are perceptually
experienced to be, sounds lack apparent intrinsic or non-relational
spatial features. Roughly, independent from spatial relations to other
sounds, experienced sounds seem to lack *internal* 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, but also are individuated in terms of inherent
spatial features.



This difference, however, does not ground an argument that any purely
auditory experience is non-spatial or that sounds fail to satisfy the
requirement on objectivity, since sounds' being experienced to
have internal, intrinsic, or inherent spatial characteristics is
necessary neither for spatial auditory experience nor to experience
sounds as objective. Since sounds phenomenologically seem to be
located in space and to bear extrinsic spatial relations to each
other, auditory experience satisfies the requirements for objectivity,
which need only secure the materials for a conception of a place for
sounds to exist when not experienced.



So, vision and audition differ with respect to space in two ways.
First, vision's spatial *acuity* surpasses that of
audition. Second, vision's objects are perceptually experienced
to have rich *internal* spatial structure, and audition's
are not. However, given the spatial characteristics 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 auditory phenomenology
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 in which sounds are individuals, sounds are not
secondary or sensible qualities. But, humans hear audible qualities,
such as pitch, loudness, and timbre, that are analogous to colors,
tastes, and scents. Thus, familiar accounts of colors and other
sensible attributes or secondary qualities might apply to the audible
qualities. For instance, pitches might be either dispositions to cause
certain kinds of experiences in suitable subjects, the physical or
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 their
perception. Thus, according to tradition, if dispositionalism,
physicalism, projectivism, or primitivism about sensible qualities is
true for features associated with one modality, it is true for
features associated with others. Despite tradition, we should be wary
to 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 (for discussion
of similar questions concerning timbre, see Isaac 2017). 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 aspects
of color perception. Each aims to establish that the colors we
perceive cannot be identified with objective physical features.
Neither argument transposes neatly to the case of pitch. Thus, we
should not assume arguments that are effective in the case of color
have equal force when applied to other sensible qualities. Color
perhaps is a uniquely difficult case.



Second, however, I discuss two respects in which pitch experience is
similar to color experience. It is instructive that these aspects of
pitch experience do raise difficulties for an objective physical
account 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 along which
tones can be ordered according to apparent "height". The
pitch of fingernails scratching a blackboard generally is higher than
that of thumping a washtub. Loudness can be glossed as the volume,
intensity, or quantity of sound. A jet plane makes louder sounds than
a model plane. Timbre is more difficult to describe. Timbre is a
quality in which sounds that share pitch and loudness might differ.
So, a violin, a cello, and a piano all playing the same note differ in
timbre. Sometimes timbre is called "tone color".



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 have
pitch. Some sounds appear to have pitch thanks to a simple,
*sinusoidal* pattern of vibration at some frequency in an object
or in the air. Some sounds appear pitched thanks to a complex pattern
of vibration that can be decomposed into sinusoidal constituents at
multiple frequencies, since any pattern of vibration can be analyzed
as some combination of simple sinusoids. Sounds appear pitched,
however, just when they have sinusoidal constituents, or
*partials*, that all are integer multiples of a common
*fundamental* frequency. Sounds with pitch thus correspond to
regular or *periodic* patterns of vibration that differ in
fundamental frequency and complexity. Simple sinusoids and complex
waveforms match in pitch (though they typically differ in timbre) when
they share fundamental frequency. This is true even when the complex
tone lacks a sinusoidal constituent at the fundamental frequency,
which is referred to as the phenomenon of the *missing
fundamental*.


#### 3.2.3 Is Pitch Physical?



A straightforward account identifies pitch with periodicity (perhaps
within some range). Having pitch is being periodic (see
O'Callaghan 2007, ch. 6). Periodicity can be expressed in terms
of fundamental frequency, so individual pitches are fundamental
frequencies. This has advantages as an account of pitch. It captures
the linear ordering of pitches. It also explains the musical
intervals, such as the octave, fifth, and fourth, for example, which
are pitch relations that hold among periodic tones. Musical intervals
correspond to whole-number ratios between fundamental frequencies.
Sounds that differ by an octave have fundamental frequencies that
stand in 1:2 ratios. Fifths involve a 2:3 relationship, fourths are
3:4, and so on. This also allows us to revise the linear pitch
ordering to accommodate the auditory sense in which tones that differ
by an octave nonetheless are the same pitch. If the pitch ordering is
represented as a helix, upon which successive octave-related tones
fall at a common angular position, 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 between *unique*
and *binary* 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 a related issue concerning pitch. Some tones with pitch sound
*simple*, while other pitched tones, such as sounds of musical
instruments, auditorily appear to be *complex* and to have
discernible components. However, the difference between audibly simple
and audibly complex pitched tones *is* captured by the simplicity
or complexity of a sound's partials. Simple tones are sinusoids,
and complex tones have multiple overtones. 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
solely occupies a distinctive place in pitch space.



Second, consider *metamerism*. Some surfaces with very different
reflectance 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 properties of
surfaces (see the entry on
 color).



Now consider the case of pitch. Are there pitch metamers? Some sounds
with very different spectral frequency profiles match in pitch. A
simple sinusoidal tone at a given frequency matches the pitch of each
complex 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 familiar from the case of color are equally
pressing when applied to the case of pitch.



First, arguments from *intersubjective variation* transpose.
Actual 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 with *spectrum inversion* in
the case of color (see the entry on
 inverted 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 pitch
ordering 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 the
*relational 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, that
red is more similar to orange than either is to green. In the case of
pitch, 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), the
*relations* 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 called
*mels*, to equal pitch intervals. The *mel* 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 the
identification 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? Compare
an 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.



In the case of pitch and other sensible features that can be put in a
linear ordering, one might respond that the relational order is
essential while the magnitudes are not. In that case, if pitch is
frequency, pitch experience has the right structure, but distorts
magnitudes of difference in pitch. This retains the periodicity theory
and explains away the results in terms of pitch experiences.



Nonetheless, Pautz (2014, 3.5) has replied that this partial error
account cannot be reconciled with certain types of possible
intersubjective difference. So, suppose instead 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 also is 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 sensible
features should just provide a physical candidate for each determinate
sensible feature, or whether the physical relationships among those
physical candidates should capture the structural relations among
sensible qualities (and, if so, which structural relations it should
capture). This is an example of how considering in detail the nature
and the experience of sensible qualities other than color promises
insights into traditional debates concerning the sensible qualities.
Pautz (2014) offers an empirically-grounded argument concerning a
variety of sensible qualities, including audible qualities, that
advances such 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 on
 the philosophy of music),
 musical experience has not been used as extensively to explore
general philosophical questions about auditory perception. This
section discusses links that are relevant to advancing philosophical
work on auditory perception.


#### 4.1.1 Acousmatic Experience



An account of listening to pure or non-vocal music should capture the
*aesthetic 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 in
music. In particular, some have argued that appreciatively listening
to music demands listening in a way that abstracts from the
environmental significance, and thus from the specific sources, of the
sounds it includes (Scruton 1997, 2-3). Such *acousmatic
listening* involves experiencing sounds in a way that is
"detached from the circumstances of their production,"
rather than "as having a certain worldly cause" (Hamilton
2007, 58; see also Hamilton 2009). Listening to music and being
receptive to its aesthetically relevant features requires not
listening to violins, horns, or brushes on snare drums. It requires
hearing sounds and grasping them in a way removed from their common
sources. Hearing a high fidelity recording thus furnishes an
aesthetically identical musical experience despite having a speaker
cone rather than a violin as source. "The acousmatic experience
of sound is precisely what is exploited by the art of music"
(Scruton 1997, 3).



This suggests an intuitive difference between music and visual arts
such as painting and sculpture. As Kivy (1991) explains, it is
difficult even with the most abstract paintings and sculptures to see
them in a way that takes them to be entirely formal or abstract. That
is, it is difficult to avoid seeing pictures and sculptures as
representational. In contrast, it seems easier to listen attentively
to the formal acoustical features of musical sounds, without being
compelled to think of what makes them.



Musical listening thus may be thought to provide a prima facie
argument against the claim that in hearing sounds one typically hears
sound sources such as the strumming of guitars and bowing of violins.
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 of *attention*.
Nothing prevents focusing one's attention on the sounds and
audible qualities without attending to the instruments, acts, and
events that are their sources, even if each is auditorily available.
That musical listening requires effort and training supports the idea
that one can direct attention differently in auditory experience,
depending on one's interests. Caring for an infant 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 of sounds themselves, but does not imply failing
to hear sound sources.



The acousmatic thesis is a limited view about which aspects of the
things one can auditory experience are aesthetically significant.
These include audible aspects of sounds themselves, but exclude, for
example, other contents of auditory experience. However, room exists
for debate over the 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 the *performance* 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., Solomon 2007).
To imagine auditorily experiencing the spatial characteristics of
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 few
philosophers have confronted it directly (Appelbaum 1999, Trout 2001a,
Matthen 2005, ch 9, and Remez and Trout 2009 are recent exceptions).
Something striking and qualitatively distinctive--perhaps
uniquely human--seems to set the perception of speech apart from
ordinary hearing. The main philosophical issues about speech
perception concern versions of the question, Is speech special? (See
O'Callaghan 2015 for a comprehensive review and discussion.)



How does perceiving speech differ from perceiving ordinary
non-linguistic sounds? Listening to music and listening to speech each
differ from listening to other environmental sounds in the following
respect. In each case, one's interest in listening is to some
degree distanced from the specific environmental happenings involved
in the production of sounds.



But this is true of listening to music and of listening to speech for
different reasons. In music, it is plausible that one's interest
is in the sounds themselves, rather than in the sources of their
production. However, speech is a vehicle for conventional linguistic
meaning. In listening to speech, one's main interest is in the
meanings, rather than in the sources of sound. Ultimately, the
information conveyed is what matters.



Nevertheless, according to the most common philosophical
understanding, *perceiving* spoken utterances is just a matter of
hearing sounds. The sounds of speech are complex audible sound
structures. Listening to speech in a language you know typically
involves *grasping* meanings, but grasping meanings requires
first hearing the sounds of speech. According to this account,
grasping meanings itself is a matter of extra-perceptual
cognition.



The commonplace view--that perceiving speech is a variety of
ordinary auditory perception that just involves hearing the sounds of
speech--has been challenged in a number of ways. The challenges
differ in respect of how speech perception is held to differ from
non-linguistic audition.


#### 4.2.2 The Objects of Speech Perception



First, consider the *objects* of speech perception. What are the
objects of speech perception, and do they differ from those of
ordinary or non-linguistic auditory perception? According to the
commonplace understanding, hearing speech involves hearing sounds.
Thus, hearing spoken language shares perceptual objects with ordinary
audition. Alternatively, one might hold that the objects of speech
perception are not ordinary sounds at all. Perhaps they are
language-specific entities, such as *phonemes* or *words*.
Perhaps, as some have argued, perceiving speech involves perceiving
*articulatory gestures* or movements of the mouth and vocal
organs (see the supplement on
 Speech Perception: Empirical and Theoretical Considerations).
 Note that if audition's objects typically include distal
events, speech in this respect is not special, since its objects do
not belong to an entirely different kind from ordinary sounds. 


#### 4.2.3 The Contents of Speech Perception



Second, consider the *contents* 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 distinctive *features*
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 to *language-specific types*, such as
phonemes, words, or other syntactic categories.



Furthermore, speech perception's contents might differ in a more
dramatic way from those of non-linguistic audition. Listening with
understanding to speech involves grasping *meanings*. The
commonplace view is conservative. It holds that grasping meanings is
an act of the *understanding* rather than of audition. Thus, the
difference between the experience of listening to speech in a language
you know and the experience of listening to speech in a language you
do not know is entirely *cognitive*.



But one might think that there also is a *perceptual* difference.
A liberal account of this perceptual difference holds that perceiving
speech in a language you know may involve hearing sounds *as
meaningful* or auditorily representing them as having semantic
properties (see, e.g., Siegel 2006, Bayne 2009, Azzouni 2013, Brogaard
2018; cf. O'Callaghan 2011b, Reiland 2015). Alternatively, a
moderately liberal account holds that the perceptual experience of
speech in a language you know involves perceptually experiencing
language-specific but nevertheless non-semantic features. For
instance, O'Callaghan (2011b) argues that listening to speech in
a familiar language typical involves perceiving its phonological
features.


#### 4.2.4 Is Speech Perception Auditory?



Third, consider the *processes* responsible for speech
perception. To what extent does perceiving speech implicate processes
that are continuous with those of ordinary or general audition, and to
what extent does perceiving speech involve separate, distinctive, or
modular processes? While some defend *general auditory* accounts
of speech perception (see, e.g, Holt and Lotto 2008), some argue that
perceiving speech involves dedicated perceptual resources, or even an
encapsulated perceptual system distinct from ordinary non-linguistic
audition (see, e.g., Fodor 1983, Pinker 1994, Liberman 1996, Trout
2001b). These arguments typically are grounded in several types of
phenomena, including the *multimodality* of speech
perception--visual cues about the movements of the mouth and
tongue impact the experience of speech, as demonstrated by the McGurk
effect (see the section
 4.3 Crossmodal Influences);
 *duplex perception*--a particular stimulus sometimes
contributes simultaneously both to the experience of an ordinary sound
and to that of a speech sound (Rand 1974); and the *top-down
influence* of linguistic knowledge upon the experience of speech. A
reasonable challenge is that each of these
characteristics--multimodality, duplex perception, and top-down
influence--also is displayed in general audition. 



> 
> See the supplement on
>  Speech 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 auditory
experience. 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 effect involves a visual
illusion as of seeing two consecutive flashes that is produced when a
single flash is accompanied by two consecutive beeps (Shams et al.
2000, 2002). Such crossmodal illusions demonstrate that auditory
experience is impacted by other modalities and that audition
influences other modalities. In general, experiences associated with
one perceptual modality are influenced by stimulation to other sensory
systems.


#### 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. Relying on information from vision
or touch could simply improve the existing capacity to perceive space,
time, or spoken language auditorily. On the other hand, coordination
between audition and other senses could enable a new perceptual
capacity. In that case, audition might rely constitutively on another
sense.



A first step in resolving this question is recognizing that crossmodal
illusions are not mere accidents. Instead, they are intelligible as
the results of adaptive perceptual strategies. In ordinary
circumstances, crossmodal processes serve to reduce or resolve
apparent conflicts in information drawn from several senses. In doing
so, they tend to make perception more reliable overall. Thus,
crossmodal illusions differ from *synaesthesia*. 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).



In the first place, the principled reconciliation of information drawn
from different sensory sources suggests that audition is governed by
extra-auditory perceptual constraints. Moreover, since conflict
requires a common subject matter, such constraints must concern common
sources of stimulation to multiple senses. If so, audition and vision
share a perceptual concern for a common subject matter. And that
concern is reflected in the organization of auditory experience. But
this by itself does not establish constitutive dependence of audition
on another sense.



However, the perceptual concern for a common subject matter could be
reflected as such in certain forms of auditory experience. For
instance, the commonality may be experientially evident in jointly
perceiving shared spatio-temporal features, or in the perceptual
experience of audio-visual intermodal feature binding. If so, some
forms of auditory perceptual experience may share with vision a common
multimodal or amodal content or character (see O'Callaghan
2008b, Clark 2011). More to the point, if coordination with another
sense enables a new auditory capacity, then vision or touch could have
a constitutive rather than merely causal impact upon corresponding
auditory experiences.


#### 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, then we might hear *as 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 visual or 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 whether there are
clear and significant distinctions among the sense modalities (cf.
Nudds 2003). Finally, multimodal perceptual experiences, illusions,
and explanatory strategies may illuminate the phenomenological
*unity* 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 on
 the unity of consciousness).
 



We can ask questions about the relationships among modalities in
different areas of explanatory concern. Worthwhile areas for attention
include the objects, contents, and phenomenology of perception, as
well as perceptual processes and their architecture. Crossmodal and
multimodal considerations might shed doubt on whether vision-based
theorizing alone can deliver a complete understanding of perception
and its contents. This approach constitutes an important
methodological advance in the philosophical study of perception (for
further discussion, see O'Callaghan 2012, 2019, Matthen 2015,
Stokes et al. 2015).


## 5. Conclusion and Future Directions



Considering modalities other than vision enhances our understanding of
perception. It is necessary to developing and vetting an adequate
comprehensive and general account of perception and its roles.
Auditory perception is a rich territory for philosophical exploration
in its own right, but it also provides a useful contrast case in which
to evaluate claims about perception proposed in the context of vision.
One of the most promising directions for future work concerns the
nature of the relationships among perceptual modalities, how these
relationships shape experience across modalities, and how they may
prove essential to understanding perception itself. Philosophical work
on auditory perception thus is part of the advance beyond considering
modalities in isolation from each other.