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



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



These descriptions of sense data focus on the basic visual properties
color and shape: the facts that we are seeing a table, a coaster, and
a daisy are not, in the usual sense-data theories, part of what is
given. Typically, the sense data are just patches with a shape and a
color; thus, if the daisy were partly obscured by a mug that had been
placed on the coaster, there would be two green shapes among the sense
data, on either side of the mug-shape, with no notion that the one
belonged with the other. Viewers typically are, of course, aware that
specific objects are present. In that case, the theory says that they
have gone beyond their sense data to perceive objects or things of
various kinds (table, coaster, flower), as opposed to the bare shape
and color of each sense datum.



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


1. In perceiving, we are directly and immediately aware of a
sense datum.
2. This awareness occurs by a relation of direct mental
acquaintance with a datum.
3. Sense data have the properties that they appear to
have.
4. These properties are determinate; in vision, we
experience determinate shapes, sizes, and colors.
5. Our awareness of such properties of sense data does not
involve the affirmation or conception of any object beyond the
datum.
6. These properties are known to us with certainty (and
perhaps infallibly).
7. Sense data are private; a datum is apprehended by only
one person.
8. Sense data are distinct from the act of sensing, or the
act by which we are aware of them.



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



It may be helpful to mention several things that are not included in
the above. Sense data are described as immediate objects of awareness,
but there is no statement that we therefore only immediately perceive
sense data and do not immediately perceive external objects, a
conclusion which many theorists accepted. Further, there is no
specification of whether sense data are two dimensional or contain
depth. Early theorists are typically read as asserting two-dimensional
 data;[2]
 the second wave, including Price (1932: 3, 218-220), explicitly
gave some of them depth. There is no statement about whether sense
data persist beyond the momentary sensing of them, as there was
disagreement on this point. There is no assertion of their origin,
causal or otherwise. It is natural to think of sense data as caused by
objects and as being related to those objects as appearance to
reality, a position expressed by some authors (e.g., Russell 1912: ch.
1; Broad 1923: 237-240). But that conception is not included
here in the agreed-upon attributes of sense data in their early
history. Similarly,
 item 5
 distinguishes the apprehension of a sense datum from that of a
material object or other entity; it takes no stand on whether such
objects are inferred from sense data, are affirmed by a cognitive act
that is not an inference, or are treated in some other way. Nor does
item 5 say whether sense data represent objects or their properties,
independently of a further act of judgment or affirmation; some
classical theorists denied that sense data are intrinscially
representational. Further, sense data were not tied to specific
neurophysiological processes, even if it was admitted by some authors
that they are conditioned by neurophysiological states (such as those
that produce afterimages). Finally, naive realists sometimes
described the immediate apprehension of a portion of the surface of an
object as a perceptual datum. Present-day taxonomies of philosophies
of perception typically contrast naive direct realism with
sense-data theory, an outlook that results in part from the present
tendency to treat sense data as mental (Crane & French 2015
[2021]). But in the classical period, there were arguments to the
effect that sense data are direct perceptions of a part of the surface
of a material object (Moore 1918-19: 23-24), a position
that Price (1932: chs. 2-3) described and sought to
undermine.



In the above statement of the Classical Notion, nothing explicit is
said about the metaphysical classification of sense data. They might
be events, substances, states of substances; they might be either
physical or mental. As mentioned, the majority of authors involved in
the original discussion held that sense data are not mental. But some
theorists advised that the term "sense data" in its basic
meaning should be treated as neutral on ontology (e.g., Moore
1913-14; Price 1932: 18-20), even if further argument led
to a view that they are not mental. For the neutralists, accepting
that sense data exist should be compatible with various theories of
perceptual ontology, such as naive realism, representative
realism, Berkeleyan idealism, and Kantian transcendental idealism.



Early theorists who considered sense data to be mind-independent
typically thought of them as persisting through time. Russell, in
early sense-data writings (1912: ch. 1), viewed such data as existing
apart from the mind as a special kind of thing (neither mental nor
physical), which was commonly designated as a *tertium quid* or
"third thing", in addition to objects (such as a physical
table) and the perceiver's mental states. Such intermediary
third things might be epistemically given only in the act of sensing
them, but they would not depend for their existence on that act. This
led to the notion of unsensed sense data (e.g., mind-independent
patches of color), which were sometimes called
"sensibilia" to indicate that they could be sensed if
someone were at the right location, but that they existed in any case
(Russell 1914b: sec. 3). In the second half of the twentieth century,
when sense data came to be viewed mainly as mind-dependent, they were
considered to be mental objects or contents of which we are directly
aware. Accordingly, the white shape of the coaster-shaped patch would
be a property of the mental object that is experienced, which was
taken to mean that the mental state itself would have among its
properties a white and roundish patch of color (Jackson 1977: chs.
4-5).



The initial positing of sense data arose within an extant discussion
of sense perception and its objects. In Oxford and Cambridge of the
late nineteenth century, it was common to discuss perception as part
of the "problem of the external world", an epistemological
problem found in modern philosophy, from Descartes onward, concerning
whether we know an external world beyond our own minds and, if so, how
we know it. These discussions typically assumed a distinction between
sensation and perception, similar in many ways to that later drawn
between sense data and the perception of material objects. A standard
description of how vision occurs would say that sensations of shape
and color are first produced via the stimulation of the sensory
nerves, and that from such sensations the perceiver forms perceptions
of objects, perhaps through a process of judgment or through learned
associative connections among sensations. Let us turn, then, to the
history of theories of sensation and perception, and the rise of sense
data.


## 2. Historical



The philosophical practice of separating what is immediately given to
vision by way of sense impression from further psychological or
logical acts of perception stems from
 antiquity.[3]
 In the optical tradition (where "optics" covered the
complete science of vision, from physics and physiology to
psychology), the medieval Islamic philosopher Ibn al-Haytham (ca.
1030) distinguished what is found in "pure sensation", the
immediate result of stimulating the sense organs, from what depends on
recognition, judgment, and reason (*Optics*, II.3.25). Arguably
(Hatfield 2020: 200), he included light, color, and visual direction
(a correlate of shape) among the things immediately sensed. The
optical tradition as promulgated in the medieval European Latin West
provided a framework for the theories of vision of Descartes and
Berkeley (Hatfield & Epstein 1979: 371). The distinction between
(a) a core of sensation and (b) perception of objects in the world as
the result of further psychological processes provided the predominant
framework for theorizing about the senses into the twentieth
 century.[4]
 The proximate background for the arguments about sense data just
after 1900 was set by engagement with early modern philosophy and
nineteenth-century British philosophy.


### 2.1 The Concept of Sense Data in the Modern Period



In discussions of early modern philosophy in the second half of the
twentieth century, Descartes, Locke, Berkeley, and Hume were sometimes
portrayed as sense-data
 theorists,[5]
 especially when suggesting that they abetted skepticism about the
external world. Indeed, the Scottish philosopher Thomas Reid diagnosed
the external-world skepticism found in Hume as arising from the theory
of ideas (initiated by Descartes and adopted by the others) being
taken by Hume to what Reid considered to be its logical conclusion
(Nichols & Yaffe 2014 [2016]). This interpretation was known to
our early sense-data theorists through study of Reid himself or via
William Hamilton's *Lectures on Metaphysics* (1861), in
which Reid's claims were discussed. In the latter half of the
twentieth century, Richard Rorty (1979: ch. 3) portrayed Descartes and
Locke as engendering a veil of perception. He did not use the term
"sense data" to describe the fabric of the veil by which
these authors isolated perceivers, but he endorsed Reid's charge
that they created a veil of ideas. This reading has been challenged
(e.g., in Yolton 1984), and the present entry qualifies it in many
respects.


#### 2.1.1 Sense Data and Early Modern Philosophy



Several threads within early modern theories of perception provide a
background for the Classical Notion of sense data. First, there was
the already-noted tendency to distinguish sensation from perception,
or a passively received sensory core from a further perceptual act.
For vision and touch, this further act typically included perception
of objects as existing and as present in a locally coherent
three-dimensional space. The act might result from judgment,
inference, or association. Second, the immediate objects of sensory
perception were sometimes rendered as mental representations from
which an external world must be inferred, possibly allowing for
skepticism about the basis for such inferences. Third, these mental
representations might or might not be attributed all or most of the
features of the Classical Notion. In particular, many early modern
theorists, as many theorists now, distinguished between representation
via intentional contents that do not literally possess the qualities
they present (such as color or shape), and representation through
entities that possess the properties (such as color or shape) as they
appear.



The terminological distinction between "sensation" and
"perception", explicitly so formulated, is usually
credited to Reid, although it can be found in Nicolas Malebranche
(Simmons 2009). Taken as a broad conceptual distinction with several
terminological and substantive variants, it is more widely present.
Descartes, in the *Meditations* (1641 [1985: 55]), speaks of
the perception of objects as arising from sensory ideas caused by such
objects. Subsequently, in the Sixth Replies, he distinguishes between
the immediate mental products resulting from stimulation in the sense
organs, nerves, and brain, such as perception of an extended area of
light and color, and subsequent inferences or calculations that yield
perception of an object with a specific size, distance, and shape
(1641 [1985: 295]). In other places, Descartes offers a physiological
account of the perception of distance that renders the third dimension
part of the sensory core (*Dioptrics*, 1637, Sixth Discourse
[1965: 105-106]; see Hatfield 2020: 212-213); even so, a
judgment would still be needed for object perception.



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



> 
> 
> We are farther to consider concerning Perception, that the *Ideas
> we receive by sensation, are often* in grown People
> *alter'd by the* *Judgment*, without our taking
> notice of it. When we set before our Eyes a round Globe, of any
> uniform colour, *v.g.* Gold, Alabaster, or Jet, 'tis
> certain, that the *Idea* thereby imprinted in our Mind, is of a
> flat Circle variously shadow'd, with several degrees of Light
> and Brightness coming to our Eyes. But we having by use been
> accustomed to perceive, what kind of appearance convex Bodies are wont
> to make in us; what alterations are made in the reflections of Light,
> by the difference of the sensible Figures of Bodies, the Judgment
> presently, by an habitual custom, alters the Appearances into their
> Causes: So that from that, which truly is variety of shadow or colour,
> collecting the Figure, it makes it pass for a mark of Figure, and
> frames to it self the perception of a convex Figure, and an uniform
> Colour; when the *Idea* we receive from thence, is only a Plain
> variously colour'd, as is evident in Painting.
> 
> 
> 



The passage makes a clear distinction between the immediate impression
caused by an object, in this case, sensation of a flat circle
variously colored, and the subsequent act by which, after a process of
learning, the faculty of judgment "frames ... the
perception of a convex Figure". In a similar vein, Berkeley
distinguishes between the immediate objects of sight, which include
light and color forming a visible extension with visible figure and
magnitude, and the mediate objects of sight, informed by touch
(*Essay Towards a New Theory of Vision*, 1709: sec. 49, 55).
The immediate object of sight for an approaching object grows in
visible extension. But, as the result of learning through the sense of
touch, the perceiver experiences the mediate object of sight, an
object of constant size. Hume offered a similar example: a table,
considered as an external object, retains a constant size; yet our
sensory image of it varies as we approach or recede from it
(*Enquiry*, 1748: XII.I.9).



A full tally would reveal that the positions of these early modern
figures are consistent with
 item 5 of the Classical Notion:



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



when the terminology is adjusted to substitute "sensory
ideas" or "immediate objects of perception" for
"sense data". They might also agree on the privacy of
sensations or sensory ideas. The authors would differ among themselves
on some other points, such as whether there is an act-object
distinction and whether sensory ideas are experienced by direct
acquaintance.



As regards external objects, Descartes and Locke were realists who
held that representations mediate the perception of external objects.
Accordingly, perceivers immediately experience sensory ideas that
present properties such as shape and color and, through an act of
judgment, come to perceive external objects as having properties that
cause those sensory
 ideas.[6]
 Berkeley and Hume sought to turn this position against them by
arguing that it has skeptical consequences. They sought to avoid such
skepticism, by adopting idealism in Berkeley's case, which makes
really existent sensory ideas the objects of perception, allegedly
avoiding skepticism, or adopting a common-sense realism about external
objects, in Hume's common-sense persona.



The fact that our early modern authors shared with the Classical
Notion a version of
 item 5
 does not make them sense-data theorists. Two key ingredients are
items 3 and 6: "Sense data have the properties that they appear
to have", and "These properties are known to us with
certainty".



As regards
 item 3,
 current consensus is that it does not represent Descartes'
position. He did not believe that states of mind such as sensory ideas
have properties of color or other sensory qualities. Rather, he held
that color and other qualities are in the mind
"representatively", or in terms of "objective
being" or "objective reality" (*Meditations*,
1641 [1985: 28-29, 74-75, 113-114]). Descartes is
here understood to be invoking a notion of intentionality or
intentional
 being.[7]
 The phenomenal red that one experiences is not an instance of the
property *red* that is predicated of the mind, but rather the
idea of red has an intentional content that is experienced as
phenomenal red (Yolton 1984: ch. 1; 1996: 74-75; Nadler 2006:
91-92). Similarly, the mind does not itself become square when
it experiences a square.



Regarding Locke, a perusal of the *Essay*, bk 2, ch. 8, reveals
that he is not clear on the matter. Interpreters find two notions of
"sensory ideas" in Locke: as objects of perception that
might potentially instantiate color and shape, and in that way be like
sense data; and as acts of perceiving that might exhibit color and
shape only by way of representation (Yolton 1984: ch. 3), understood
here to mean nonconceptual intentional content (see
 note 7).



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



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



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



he answered:



> 
> 
> those qualities are in the mind only as they are perceived by it, that
> is, not by way of *mode* or *attribute*, but only by way
> of *idea*; and it no more follows, that the soul or mind is
> extended because extension exists in it alone, than it does that it is
> red or blue, because these colours are on all hands acknowledged to
> exist in it. (*Principles*, pt 1, art. 49; see also *Three
> Dialogues*, Third Dialogue [2008: 229-230])
> 
> 
> 



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



As regards
 item 6,
 that perceivers enjoy certainty in the apprehension of sensory ideas,
a standard view has Descartes and Locke asserting or seeking such
certainty (Rorty 1979: chs. 1, 3). This reading has been challenged
for Descartes, Locke, and Berkeley (Yolton 1984: ch. 5; Hatfield
2011). Accordingly, the real hope for a prominent early modern
conception that approaches the Classical Notion, especially as regards
items 3 and 6, lies with Hume.



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



In *An Enquiry Concerning Human Understanding* (1748), Hume
considers the common-sense view that we directly perceive material
objects, such as a table. This sort of naive realism is, Hume
says,



> 
> 
> destroyed by the slightest philosophy, which teaches us, that nothing
> can ever be present to the mind but an image or perception, and that
> the senses are the only inlets, through which these images are
> conveyed. (*Enquiry*, XII.I.9)
> 
> 
> 



He then argues:



> 
> 
> The table, which we see, seems to diminish, as we move farther from
> it: But the real table, which exists independent of us, suffers no
> alteration: It was, therefore, nothing but its image, which was
> present to the mind.
> 
> 
> 



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


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



Hume's own conclusion, quoted above, is that we perceive only an
image of the table. This conclusion arises from the larger context in
which our immediate experiences are sense impressions (a version of
 item 1).
 Because the object seen diminishes but the real table does not, our
sensory experience does not acquaint us with the table but with
something else, a mental or phenomenal
 impression.[9]
 As a result, Hume held that we cannot know an external world of
mind-independent objects, such as the purported "real
table", but must restrict our sensory knowledge to sequences of
sense impressions.



Thus far, we have not found explicit endorsement of items 3 and 6.
These items are found most clearly in Hume's *Treatise of
Human Nature* (1739-40). He there says that "every
impression", including both internal and external, and hence
including sense impressions, "appear, all of them, in their true
colours, as impressions or perceptions" (*Treatise*,
I.4.2.7; again, Chisholm's epistemic "appear"). But,
further,



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



Otherwise, we must suppose



> 
> 
> that even where we are most intimately conscious, we might be
> mistaken.
> 
> 
> 



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



> 
> 
> the very idea of extension is copy'd from nothing but an
> impression, and consequently must perfectly agree to it. To say the
> idea of extension agrees to any thing, is to say it is extended.
> (*Treatise*, I.4.5.15)
> 
> 
> 



An impression of a square, which we are conscious of as extended, is
really extended. Our consciousness of sensory properties, as those
properties are present to us phenomenally, reveals the properties as
they are. We can't be mistaken about them. Items 3 and 6 are
affirmed by Hume. Indeed, substituting "impression",
"idea", and "perception" for "sense
datum", we might ascribe to Hume all of the items in the
Classical Notion, save perhaps
 item 8,
 the act-object relation. In reading the *Treatise* and
*Enquiry*, one might be tempted to ascribe item 8 to Hume, but
he notoriously rejected such ascriptions in the Appendix to the
*Treatise*.



The need for philosophers to address the problem of the external world
was widely regarded as a legacy of early modern philosophy to the
nineteenth and twentieth centuries. At Cambridge and Oxford into the
1890s, discussions of the problem of the external world referenced
Hamilton's *Lectures on Metaphysics* (1861) and J. S.
Mill's *An Examination of Sir William Hamilton's
Philosophy* (1872). In that decade and the next, G. F. Stout and
James Ward at Cambridge discussed the problem as did Thomas Case and
John Cook Wilson at Oxford (Nasim 2008, Hatfield 2013b).


#### 2.1.2 Hamilton, Mill, and Sense Data



Hamilton was Professor of Logic and Metaphysics at Edinburgh, and in
that capacity regularly gave lectures in each subject area. In his
*Lectures on Metaphysics*, he offered a taxonomy of positions
on the problem of the immediate objects of perception. This taxonomy
distinguished those who accepted or rejected what Hamilton considered
to be the fundamental "fact of consciousness" or
"datum of consciousness" (1861, 2: 86). In
Hamilton's view,



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



Further, it is "given" that ego and non-ego are mind and
matter and that these are co-equal. Hamilton's preferred
position, which he called "Natural Realism" or
"Natural Dualism", affirmed the datum of consciousness and
held that some portion of the external world is given to the subject
in immediate perception. He argued that, in vision, the physical
retinal image is the immediately given material entity (1861, 2: 130,
179-184). Accordingly, we have intuitive knowledge of a
two-dimensional image, which we are able to understand as a projection
of the three-dimensional physical world. This position would allow
Hamilton to reject Hume's Argument from Perceptual Variation,
because the "real object" of perception, the retinal
image, does vary as we approach or recede from the real table. On this
view, we don't immediately perceive the real table, but we infer
it from a real item of the external world that we do perceive
immediately: the retinal image.



Hamilton described positions that emended or rejected his fundamental
datum of consciousness. Some rejected the fundamental datum and held
that in perception we are immediately aware of something that differs
from the external world. Hamilton called this a
"representative" theory (1861, 1: 295), or, more
impressively, he named it "Hypothetical Dualism" or
"Cosmothetic Idealism". Holders of this position are
subdivided



> 
> 
> into those who view in the object immediately perceived, a *tertium
> quid* different both from the external reality and from the
> conscious mind, and into those who identify this object with a
> modification of the mind itself. (1861, 1: 298)
> 
> 
> 



Hamilton effectively invoked a notion of sense data, even if the
closest he came terminologically was to speak of the
"datum" of consciousness. He accepted most tenets of the
Classical Notion, but exceeded that notion in considering the datum to
be the physical retinal
 image.[10]
 He further described positions opposed to his own that are close to
positing a sense datum, either as a *tertium quid* or as a
mental representation. He is not clear on whether these purported
representative entities would be ascribed the properties they seem to
have
 (item 3),
 but his emphasis on the power of introspective or intuitive
consciousness would move him in that direction.



On one way of reading his work, John Stuart Mill offered arguments
against some immediately given sense data before they had been so
named. Mill did not deny Hamilton's premise that whatever is
immediately and intuitively known in consciousness is known with
certainty (*Examination*, 126), but he contended that some
states of consciousness that seem intuitive, such as the allegedly
intuitive experience of a material world, are actually the result of
previous psychological processes, which build up habits of mind
through association. They would accordingly be complex products, not
intuitively given primary data.



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



> 
> 
> The test by which they all decide a belief to be a part of our
> primitive consciousness--an original intuition of the
> mind--is the necessity of thinking it. Their proof that we must
> always, from the beginning, have had the belief, is the impossibility
> of getting rid of it now. This argument, applied to any of the
> disputed questions of philosophy, is doubly illegitimate: neither the
> major nor the minor premise is admissible. For, in the first place,
> the very fact that the questions are disputed, disproves the alleged
> impossibility. Those against whose dissent it is needful to defend the
> belief which is affirmed to be necessary, are unmistakable examples
> that it is not necessary. It may be a necessary belief to those who
> think it so; they may personally be quite incapable of not holding it.
> But even if this incapability extended to all mankind, it might merely
> be the effect of a strong association. (*Examination*,
> 143-145)
> 
> 
> 



Finding that a belief has been entrenched through past experience
unmasks it as merely habit-based and not a product of
necessity-bestowing intuition. What seems to be a primitive datum
might actually result from habit. We'll return to this
distinction below, anent Russell's logical and psychological
"hard data".


### 2.2 Formation and Flourishing of the Concept and Term "Sense Data"



Formation of the concept and the term "sense data" is
standardly attributed to G. E. Moore and Bertrand
 Russell.[11]
 The background to this development includes not only the problem of
the external world but also their early and joint commitment to
British Idealism. The affirmation of sense data denied a central tenet
of such idealism, that perception yields no knowledge because it is
always an incomplete grasping of the whole or the Absolute. As Russell
put it, looking back from 1943:



> 
> 
> Bradley argued that everything common sense believes in is mere
> appearance; we reverted to the opposite extreme, and thought that
> *everything* is real that common sense, uninfluenced by
> philosophy or theology, supposes real. With a sense of escaping from
> prison, we allowed ourselves to think that grass is green, that the
> sun and stars would exist if no one was aware of them, and also that
> there is a pluralistic timeless world of Platonic ideas. (1951:
> 12)
> 
> 
> 



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



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


#### 2.2.1 Moore and Sense Data



Moore broke with idealism in 1898 and his "Refutation of
Idealism" appeared in 1903. This paper did not introduce the
concept of sense data, but it formulated the distinction that made him
a realist, which he maintained throughout his life, between the act of
sensation and its object. Moore contended that every case of sensation
involves awareness of two distinct elements, one common to all
sensations and the other differing among them. He distinguished the
common element, which he called "consciousness", from the
differing "objects" of sensation, which might be blue in
one case, green in another. He did not claim to prove that the object
is what it seems to be: a material object at a distance, but he
endorsed a naive realism, according to which we are directly
aware of "the existence of a table in space" just as we
are directly aware of the conscious element (1903: 453).



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



> 
> 
> By sense-data I understand a class of entities of which we are very
> often directly conscious, and with many of which we are extremely
> familiar. They include the colours, of all sorts of different shades,
> which I actually see when I look about me; the sounds which I actually
> hear; the peculiar sort of entity of which I am directly conscious
> when I feel the pain of a toothache, and which I call "the
> pain"; and many others which I need not enumerate. But I wish
> also to include among them those entities called "images",
> of which I am directly conscious when I dream and often also when
> awake; which resemble the former in respect of the fact that they
> *are* colours, sounds, etc.; but which seem, as a rule, like
> rather faint copies of the colours, sounds, etc., actually seen or
> heard, and which, whether fainter or not, differ from them in respect
> of the fact that we should not say we actually saw or heard them, and
> the fact that they are not, in the strictest sense of the words,
> "given by the senses". All these entities I propose to
> call sense-data. And in their case there is, of course, no question
> whether there *are* such entities. The entities meant certainly
> *are*, whether or not they be rightly described as
> "sensations", "sense-presentations",
> "sense-data", etc. Here the only question can be, whether
> they are "mental". (1909-10: 57)
> 
> 
> 



Moore's remarks are not in the spirit of *arguing* that
sense data exist but of *drawing our attention* to their
obvious existence; his argument concerns whether they are mental.
Moore observes (1909-10: 58) that philosophers such as Stout
regard sense data as mental because they fail to distinguish act from
object, consciousness of blue from the blue of which we are conscious.
Having made this distinction between act and object, he finds it
manifest that the object is non-mental (1909-10:
58-60).



Moore intended the term "sense data" to provide a neutral
description for whatever is the immediate object of our awareness and
hence to be neutral on the problem of the external world. Over the
next half century, he considered various theories about how sense data
might relate to physical objects. In these discussions, he sometimes
used "sensibles" as a broader term that includes unsensed
sense-data (1913-14: 357-358). Without following the
various shifts in his preferred theory, we can note that he considered
five relations that sense data might have to physical objects or other
actual or potential causes (Moore 1913-14, 1918-19, 1925,
1957). On one conception, physical objects might be regarded as


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



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


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



Finally, a sense datum might be


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



Moore (1913-14: 376) identified (1) with Russell and Mill (e.g.,
Russell 1914a, b) and allowed that it might be right. But he favored
positions (4) and (5). In (4), sense data stand in a relation to
physical objects, called "relation R", which turns out to
be a representational relation (Moore 1925, 1957). In (5), the
perceiver is directly aware of a part of the surface of a physical
object.



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



Position (4),
 a form of representative
 realism,[14]
 accounts for the perceptual relativity that makes us want to
distinguish the immediate object of perception from the properties we
ascribe to physical objects (Perceptual Variation). The immediate
object is elliptical; the penny, round. Although accommodating
relativity, this position may be challenged for making it difficult to
know the properties of the physical object, for the object is once
removed from perception. But Moore (1913-14: 379-380) was
not swayed by this challenge, generally aligning himself with common
sense beliefs about physical objects.



Position (5)
 equates the sense datum with a portion of the surface of the physical
object. In order to account for perceptual relativity, Moore
(1918-19: 23-24) now violates a basic tenet concerning
sense data: that we perceive them as they are
 (item 3 of the Classical Notion).
 If the penny is round and we are directly aware of its physical
surface, even when viewed at an angle, we are perceiving a round
thing. But, Moore holds, the round thing is perceived to
"seem" elliptical. It is not perceived to *be*
elliptical, as in the other four positions, and as Moore himself had
previously assumed (1918-19: 23). Hence, in holding (5), one
must distinguish the actual properties of the directly seen surface
from the properties it seems to have (1918-19: 24). There is no
actually elliptical datum; rather, the circular datum simply seems to
be elliptical. Moore did his best to uphold the direct realism of (5)
and at times favored it. But in his final publication (1957), he
endorsed
 (4)
 instead.


#### 2.2.2 Russell and Sense Data



In 1911-1912, Russell returned to a project he had started soon
after the break from Idealism, an investigation of the concept of
matter. A need to first secure the foundations of mathematics had
diverted him from this project (and resulted in *Principia*).
Now, from 1912 on, he approached the analysis of matter from the
perspectives of epistemology and ontology, investigating our knowledge
and conception of the fundamental constituents of the external world.
The main events concerning sense data proper run from the *Problems
of Philosophy* (1912) to the *Analysis of Mind* (1921),
although the legacy of sense data extends further.



Russell's initial discussions in this period made epistemology
primary. Consider the opening sentence of his 1912 popular book,
*The Problems of Philosophy*:



> 
> 
> Is there any knowledge in the world which is so certain that no
> reasonable man could doubt it? (1912 [1997: 7])
> 
> 
> 



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



Russell finds that when he looks at an object such as a table, his
perception of the table does not reveal the actual properties of the
table but rather a series of sensations that vary in what they
present. The character of color sensations change as one moves around
a table, sometimes being brown, but in cases of reflected light (say,
from a window), giving a sensation of white. The shape of the table
appears as a sequence of quadrilaterals; viewed from one end, it
appears as a regular trapezoid, and the physically parallel sides
yield a sense experience of convergence. Russell argues for the
existence of sense data with a version of the Argument from Perceptual
Variation, which, for shape, runs as follows (1912 [1997:
10-11]):


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



He offers similar arguments for color and felt properties such as
hardness.



Russell gives the name "sense data" to



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



In further discussion, he affirms or implies the eight elements of the
Classical Notion. He further suggests that sense data may exist only
when sensed (1912 [1997: 23]). Like Moore, he emphasizes the
importance of distinguishing between the mental act of sensation and
the object of that act (1912 [1997: 12]). The act is mental, the
object need not be. Like Moore as well, he suggests that those who
believe that sense data must be mental have failed to distinguish act
from object (1912 [1997: 41-43]).



Russell further asks whether the table is real and, if so, what it
might be (1912 [1997: 11, 19-25]). He answers that the table
most likely is a real material object, rectangular and brown.
Accordingly, he assigns to material objects the role of causing sense
data. Sense data then "represent" the objects that cause
them (1912 [1997: 23]). This is not part of the base concept of sense
data but is the result of further analysis. He has posited sense data
as "third things", standing between material objects and
perceivers, and serving as the immediate objects of perception.



In *Problems*, Russell distinguished knowledge by acquaintance,
which involves a direct relation to sense data, from knowledge by
description or "knowledge that". Although bare
acquaintance makes us aware of the properties of sense data, it does
not assert any truths or involve any judgments, hence does not
constitute "knowledge that" something is the case (1912
[1997: 46]). We come to know basic facts about sense data by making
judgments about them. In obtaining sense data of a brown table, we
easily come to know that we are seeing something brown, or,
subsequently, a whitish sheen, or an appearance of one or another
trapezoidal surface. This is what Russell calls "knowledge of
things by *description*" (1912 [1997: 46]). Description
is only possible if we have access to universals, such as brownness,
whiteness, being a surface, being a trapezoid, and so on. According to
Russell, repeated sensations allow us to become acquainted with
sensory universals:



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



Having acquaintance with universals such as whiteness, we can make
judgments that yield knowledge by description of the properties of
things. These are judgments of fact about sense data, which establish
truths.



Going forward, Russell retained the view that sense data (or their
counterparts) provide epistemological bedrock. But within a year or
so, in *Our Knowledge of the External World* (1914a) and
"The Relation of Sense-data to Physics" (1914b), he
changed position on the metaphysics of sense data in relation to
material objects. He came to view such objects as logical
constructions out of sense data. He found no need to assert the
existence of material objects, whether the substantial
"thing" of common sense or the atoms and molecules of
physics (1914b: 109, 114-115). Rather, he populated the world
with minds and sense data and new entities termed
"sensibilia". Things and micro-physical particles became
logical constructions, not inferred entities as in his representative
realism of 1912. This adjustment may be seen as an extension of his
recognition that sense data are very well known. Treating material
objects as mere logical constructions avoids a shaky inference from
well-known data to the actual existance of physical things.



During this development, Russell retained and clarified his conception
of occurrent sense data, including the eight points of the Classical
Notion. He retained the distinction between mental act and non-mental
object (1914a: 76; 1914b: 113). Sense data are non-mental, but they
exist as sense data only while they are present to the mind (1914a:
64; 1914b: 110). He now identified two additional entities, inferred
rather than constructed, that would figure into his account of the
external world: other people's sense data, inferred from their
testimony and ultimately relying on analogy with one's own mind
to posit other minds; and



> 
> 
> the "sensiblia" which would appear from places where there
> happen to be no minds, and which I suppose to be real although they
> are no one's data. (1914b: 116)
> 
> 
> 



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



> 
> 
> I shall give the name *sensibilia* to those objects which have
> the same metaphysical and physical status as sense-data without
> necessarily being data to any mind. (1914b: 110)
> 
> 
> 



The notion of "object" here is not that of an ordinary
physical object. Rather, as the objects of sensation, Russell posits a
bevy of fleeting particulars, the exact time course of which he
didn't specify.



Russell's analysis of our perception of an external world
advanced in other ways. In *Problems* (1912: ch. 1), he had
noted that we are so accustomed to judging the "real"
shapes of things that we believe "that we actually see the real
shapes" (1912 [1997: 10]), that is, we uncritically assume that
we see the table as rectangular. But when we adopt the attitude needed
to produce a drawing of the table, we may come to notice the
appearances of various trapezoidal shapes. In *Our Knowledge*
(1914a: ch. 3), Russell further analyzes this difference. His analysis
depends on the relation between what is logically and psychologically
primitive or else derivative. Logically primitive beliefs are those
that are not logically derived via inference or construction. In order
to strengthen our knowledge, we seek to align these primitive beliefs
with the psychologically primitive. While having an ordinary
perception of the table with its actual shape, we mistakenly believe
that this experience, which has resulted from a learned psychological
process, is psychologically primitive; accordingly, we also designate
this perception of the table as logically primitive. Scientific
psychology now offers a point that Russell had attributed to learning
to draw: that what is actually psychologically primitive is the
perspectival view of the table as trapezoidal (1914a: 68). Armed with
this psychological finding, we endeavor to make the trapezoidal datum
logically primitive; what is psychologically primitive is more secure
and so provides us with "hard data" by comparison with the
"soft data" of the common-sense belief about the
table's shape (1914a: 69-70). As with Mill's
apparently but not actually intuitive beliefs, before epistemological
analysis many beliefs about sensory things are logically primitive,
because not reached through logical derivation, and psychologically
derivative, because caused by extra-logical psychological processes
such as association. Upon further consideration, we favor beliefs that
are both psychologically and logically primitive (1914a: 69-70)
and so establish sense data as logically primitive.



Russell's postulation of sensibilia allowed him to gain in
metaphysical regularity and economy. Sensibilia make it that the
objects of sense, color patches and like, do not simply go out of
existence when unobserved. Rather, sensibilia persist and we sample
them when we occupy positions that present specific
table-perspectives. But, further, he economizes by replacing the
"real" physical table with sense data and sensibilia at
various locations. These are coordinated between the senses, and so
include those as of approaching the table with our eye until our face
touches it, a perspective that connects with the felt location of
table (1914b: 119-120).



By 1920, Russell proposed even greater ontological economy by giving
up mental acts and the experiencing subject as distinct from sense
data. This meant that the "data" were no longer given to a
distinct subject and so were improperly named as "sense
data"; henceforth, he spoke of "momentary
particulars" or "events". He explicated this
"neutral monism" in the *Analysis of Mind* (1921:
ch. 1). He saw himself as adopting James's (1904) position,
according to which both the self and the external world are composed
of one neutral stuff--momentary particulars, characterized by
perceptual qualities such as color and shape (for visual particulars).
Accordingly, he discarded the act-object analysis of mind and sense
perception, as there is no longer recognition of a mind that acts but
only of various sequences of momentary particulars. The sequence that
includes all the experiences that I've had today (and before)
constitutes my mind, another sequence of such particulars constitutes
your mind. Physical objects, such as a table, become a different
sequence of particulars, which are like sensibilia in giving
perspectives on a table or a room that is composed of them. If no
one's sequence of particulars intersects with those of a
particular classroom, then that room and its contents consist of a
series of momentary particulars now occurring without anyone present.
The material entities of physics are, as before, treated as
constructions from momentary
 particulars.[15]


#### 2.2.3 Further Development of the Classical Notion of Sense Data



Many other classical theorists accepted the view that sense data are
non-mental. Broad (1923) and Price (1932) were realists who
acknowledged both sense data and material objects. Broad explored a
"critical scientific realism" by elaborating the notion of
sense data (or, in his terms, "sensa") from Russell and
others. His position was in some ways similar to traditional
representative realism, but Broad denied that sensa are themselves
mental states, in the sense of being dependent on the mind for their
existence (1923: 265-266), and, further, he denied that physical
objects are inferred from sensa. Rather, sensing sensa induces an
underived belief in mind-independent physical objects, a belief that
is not reached by inference and cannot be justified by inference
(1923: 268). Broad agreed with Russell's point that, in ordinary
perceptual consciousness, sense data are not manifest but may become
so through an act of attention. He rejected the picture that we first
focus on sensa and then explicitly infer objects and their properties
from them; rather, we "pass automatically", by a primitive
feature of our psychology, from sensa (perhaps unnoticed) to judgments
about objects and their properties (1923: 246-248).



Price's book, *Perception* (1932), is an especially
probing account of sense data, their ontological status and cognitive
role. He begins by isolating a sense datum as what is given in
perception such that we cannot doubt its existence: the extant bulgy
patch of red that occurs when a tomato is viewed in good light cannot
be doubted, even if we can doubt whether we are seeing a tomato or
indeed any physical object at all (1932: 3). The bulgy patch is
directly present in consciousness, that is, is "given" as
a datum by means of a relation of mental acquaintance. Price, as has
been mentioned, recommended that the term "sense data"
should be considered, in its meaning, neutral on ontology. But he went
on to argue for a specific conception of what sense data are,
including a specification of their cognitive status and their ontology
as a *tertium quid*.



Against the "causal" theory of perception (effectively,
representative realism), Price argues that the concept of cause is
neither necessary nor sufficient for our belief in physical objects,
that even in spatial perception only in some cases do sense data
resemble physical objects, and that in any case material objects are
not inferred from sense data. He further maintains that although sense
data are distinct from material objects (contra naive realism)
and their existence may require the existence of minds, they are not
mental (1932: 117-127, 316-317). He elaborates notions of
perceptual acceptance and perceptual assurance to describe the
non-inferential cognitive acts through which we accept and believe
that material objects exist. The belief in material objects consists
in (a) the construction of a "standard solid" from sense
data, which is a spatial structure that fits the outer shape of a
physical object in three dimensions, together with (b) belief in a
causally efficacious "physical occupant" of this location
(1932: ch.
 8).[16]



Price (1932: ch. 2, esp. pp. 27-33) gave an early formulation of
the renowned Argument from Illusion as supporting sense data in
opposition to naive realism. In this argument, if the penny looks
elliptical or the table trapezoidal, these are illusions, since the
penny is round and the table rectangular. Price's version of the
Argument may be formulated as
 follows:[17]


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



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



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



A. J. Ayer (1940), contra Price and others, argued that the existence
of sense data is not a factual question but a matter of linguistic
convention. He held that all theorists of perception might agree on
the phenomenal facts about seeing pennies but disagree on the
implications for the real shapes of pennies. One theorist holds that,
as the observer changes position, the penny appears elliptical but
stays round; the theorist endorses sense data (by Illusion). Another
theorist agrees about the series of elliptical appearances with a
moving observer but holds that, when the penny looks elliptical, it
has changed shape to become elliptical (Ayer 1940: 17-18).
According to Ayer, the competing theories can't be
differentiated empirically (they agree on the elliptical appearances)
and so they become recommendations about perceptual language. Ayer
(1940: ch. 1) found that the Argument from Illusion supports choosing
sense-data language. He developed the language of phenomenalism, in
which statements about material objects are understood as statements
about actual and possible sense data (1940: ch. 5). In his scheme,
even though sense data do not exist unsensed (possible sense data are
merely hypothetical), they need not be classed as mental states (1940:
76-78). Ayer (1940: 61-65) departed from the other main
theorists in rejecting an act-object analysis of sensations and their
objects
 (item 8).


## 3. Major Critical Responses or Oppositions



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



Austin's lectures in Oxford (from 1947 on), critical of sense
data, were published in 1962 as *Sense and Sensibilia*. They
took aim at the doctrine that



> 
> 
> we never see or otherwise perceive (or 'sense'), or anyhow
> we never directly perceive or sense, material objects (or material
> things), but only sense-data. (1962: 2)
> 
> 
> 



Austin accuses Ayer (1940), Price (1932), and others of embracing a
"scholastic" view that analyzes a few poorly understood
words and "half-studied 'facts'". By contrast,
he promises to show



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



Austin's criticisms focus on language use, even while dissenting
from Ayer that the issues are entirely terminological (Austin 1962:
ch. 6). He focuses on the "argument from illusion" from
Ayer (also citing Price), and complains that ordinary cases of
perceptual relativity and variation, such as the elliptical-looking
penny, are not normally taken to be illusions. In fact, as we have
seen, many earlier sense-data theorists would agree. Austin has little
to say about the Argument from Perceptual Variation from the classical
discussions. He finds the penny example to be atypical because pennies
have stable, sharply bounded shapes whereas many objects, such as
cats, do not. In considering the Argument from Illusion, Austin (1962:
52) objects to the continuity premise, which we have just seen in
Price: that if, in one case, we explain a mismatch between object and
perception by introducing a special entity such as a sense datum, we
must then accept that in phenomenally similar cases we can't
have direct, veridical perception. (The continuity premise remains in
dispute today, see Crane & French 2015 [2021].) More generally,
sense-data theorists had argued that in describing elliptical
appearances gained from a penny, one adopts a special sense of
"see" that (1) does not require that an elliptical
material object exists, but (2) does affirm that an elliptical sense
datum exists (1962: ch. 3). Austin's denial that something
elliptical must be seen in such cases draws on ordinary language. But
Price and others distinguished different senses of perception words
advisedly; Austin fails to engage their reasons more directly.
Further, he does not consider theorists who say that we perceive
material things as a result of sensing sense data (e.g., Broad 1923:
248).



Another challenge to sense-data theory sought to reveal a
"sense-datum fallacy" (Chisholm 1957: 151-152). The
charge is that sense-data theorists make inferences like this:
something appears elliptical, therefore what we see is an appearance
that is
 elliptical.[20]
 The fallacy consists in simply assuming that if something appears a
certain way (such as Price's flat circle produced by a spherical
ball), something must exist that is that way (a sense datum). Critics
then argue that, in asserting that something appears elliptical (or
flat), one might simply be describing the character of the appearance,
not asserting that an item with an elliptical shape (or flat surface)
is present. Afterall, we normally do accept that things need not be as
they appear. In response, it has been contended that sense-data
theorists do not simply move from "appears elliptical" to
"is elliptical" (Smith 2002: 35-36). Rather, the
positing of sense data most commonly arises through acceptance of an
act-object analysis of perception
 (item 8),
 which also affirms that the objects of perception are directly
apprehended
 (item 1)
 and that they are as they appear
 (item 3).
 The arguments from Variation and Illusion may then be used to contend
that we often experience perceptual objects whose properties are
distinct from the properties ascribed to material objects, as in the
penny appearing elliptical, and that from the qualitative similarity
of illusions (or variant appearances) and alleged direct perceptions,
we should conclude that we are always acquainted with sense data. Such
arguments may indeed be challenged, but there has been no simple and
unwary move from appears such to is such.



Theorists who sought to avoid this alleged fallacy held that a penny
might appear elliptical in a phenomenally direct way without an
elliptical item being present to the mind. These are sometimes known
as "appearance theories". One such position is the
adverbial analysis, which holds that phenomenal qualities inhere in
the mental activity of perceiving but without being present as objects
or as property-instances. Accordingly, we perceive the penny itself,
but perceive it in a certain manner: roundly, or elliptically, and
redly (Ducasse 1942; Chisholm 1957: part 3). To say that we are
experiencing the penny elliptically is to say that we enjoy an
elliptical appearance without there being an actually elliptical item
present to us; rather, the appearance is understood simply as our
manner of sensing the penny, or as the way our sensory activity makes
us experience the shape of the penny. A related sort of appearance
theory renders the elliptical appearance as a mental content, or an
intentional state, with the contents elliptical and red (Barnes
1944-1945). Accordingly, via intentional content, one can
experience an elliptical spatial structure without being acquainted
with an actually elliptical object. The intentional content
instantiates an appearance as a state of the perceiver, whose
perceptual experience thereby represents the world as being a certain
way, for instance, as containing an elliptical shape. (Intentionalist
theories are found in Dretske 1995 and Tye 1995; an important earlier
formulation is Anscombe 1965.) These two types of theories account for
Variation and Illusion via adverbial or intentional contents that can
also account for veridical cases of perception. Indeed, these theories
can allow that, in some instances (with moderate slant), we see a
penny that is round by seeing it as being round and slanted relative
to the line of sight. Adverbialists would say that we see the penny
roundly; intentionalists, that we see the penny via mental content
that represents it as being round. (On these philosophies of
perception, see Crane & French 2015 [2021].)



There is a tradition in philosophy and psychology that denies, on
phenomenal grounds, the distinction between impoverished, typically
two-dimensional sensory contents or states and the perception of
objects in three dimensions. One can see William James's
affirmation that sensations are intrinsically world-directed (1890, 2:
1-3) and exhibit three-dimensional volume (1890, 2:
135-136) as objecting in advance to the Classical Notion of
sense data. James denies
 item 5,
 that sense data are neutral about a world beyond them, in favor of a
position that both sensation and perception give us "an
immediately present outward reality" (1890, 2: 2). The Gestalt
psychologists (Koffka 1935: chs. 1-3; Kohler 1929 [1947:
chs. 1-3]) also adopted a position in which we are perceptually
given a world, not an impoverished content or object distinct from the
world. They made perception of the valence or functional values of the
world into an immediately given aspect of perceptual content. They
also denied that two-dimensional sensations or sense data are the core
from which perceptions are developed: the world is given in
 three-dimensions.[21]
 The Gestalt theorists drew attention to the phenomena of size and
shape constancy. If two objects of the same size are presented at five
and ten feet, the retinal image of the farther object will be half as
large (in linear height) as the image of the nearby object; yet
observers report them to be phenomenally of the same size (or nearly
so). Similarly, a circle seen at a (moderate) slant projects an
ellipse on the retina but appears circular. These theorists contended
that significant phenomenal differences in size between the two
objects, and the appearance of an elliptical shape, are produced only
under special laboratory conditions or through trained acts of
attention, as might be found in those who are taught to draw (Koffka
1935: 222-223; Kohler 1929 [1947: 71-74]).
Subsequently, the perceptual psychologist James J. Gibson (1950)
agreed with the Gestalt theorists in arguing that the two-dimensional
experiences that are like sense data arise from acts of attention that
artificially alter our phenomenology away from the three-dimensional
visual world that we normally perceive. Accordingly, the
two-dimensional visual field is not an immediate object of perception
or primitive constituent of perception that is uncovered by attention.
Rather, it is constructed through a special effort of attention.
Gibson held that, in everyday perception of a circle at a slant (say,
your neighbor's plate at a dinner party), the object looks like
a circle at a slant and so looks circular, not elliptical (1950: chs.
1, 9). In subsequent writings, Gibson (1966, 1979) developed the
notion that sensory systems are attuned for presenting to perceivers a
navigable environment and its affordances for meeting the
perceiver's needs and goals.



Each of these three theoretical stances denies, on phenomenal grounds,
the two-fold character of perceptual consciousness, or the division
into impoverished sensuous element and accompanying interpretation or
object-positing. Rather, sensory experience is intrinsically
world-presenting, spatially expansive (in three dimensions), and
utility alerting.



Wilfrid Sellars sought to unmask the "myth" of a pure
given to which subjects can respond atomistically in a way that yields
an epistemic given (a primitive basis for knowledge). According to
Sellars's highly regarded "Empiricism and the Philosophy
of Mind" (1956), the "classical" sense datum
position holds that "s is red" can be known
non-inferentially. Classical sense-data philosophers



> 
> 
> have taken givenness to be a fact which presupposes no learning, no
> forming of associations, no setting up of stimulus-response
> connections. (1956: SS6)
> 
> 
> 



They accept an "inconsistent triad":



1. *X senses red sense-content s* entails *x
non-inferentially knows that s is red*.
2. The ability to sense sense-contents is unacquired.
3. The ability to know facts of the form *x is ph* is
acquired.



A and B together entail not-C; B and C entail not-A; A and C entail
not-B. (1956: SS6, emended)




Who allegedly subscribed to this triad? Some of Sellars's
examples draw from early modern philosophy; he effectively attributes
a version of A and B to Locke, Berkeley, and Hume (1956: SS28)
but, it seems, not C. He criticizes Ayer's linguistic analysis
(1956: SSSS8-9) but suggests that it is aimed more at
analyzing object-talk into sense-data talk than at showing sense-data
talk to be epistemically basic (as in A). He briefly alludes to Broad
(1956: SS11) and Price (1956: SS31), without ascribing the
triad to them. He also devotes considerable attention further on to
the role of an ostensive "given" within the
"positivistic conception of science" (1956: SS43), and
it is tempting to surmise that his main target was that conception as
put forward by Carnap, Reichenbach, and others. To assess that
direction, we would need to interpret the positivist shift from
elementary experiences to protocol sentences as epistemically basic,
which would take us far afield (but see Leitgeb & Carus 2020).



In relation to the triad, a prominent reading of Locke has him impute
epistemic authority to sensory ideas as giving knowledge of their
causes (Ayers 1991: vol. 1, ch. 18). Locke is attributed A and B but
not C. It is doubtful that Descartes, who distinguished sensing from
judging, would endorse A. Among the classical sense-data theorists,
many would deny A, distinguishing sensing and
"acquaintance" from "knowing that"; sensing
red may usually and non-inferentially lead to knowing that s is red,
but not as a simple matter of entailment. Russell explicitly denied A;
sensing may be unacquired but "knowing that" involves
judging, not mere sensing, and the universals that enter judgments of
sensory quality (such as whiteness) are acquired, affirming C. Sellars
himself rejects A, complicates B, and affirms C, adopting (by
assumption) a position of "psychological nominalism"
(1956: SS29), according to which kind-terms, including color
terms, and the ability to be aware of color instances are learned as
part of a linguistic and conceptual web of meaning (holism).



In his holism about color terms, Sellars raised an objection that
would engage Russell's conception of atomic statements about
sense data as epistemically basic (Russell's psychologically and
logically primitive hard data). Sellars (1956) constructs an elaborate
myth of his own, in which our behaviorist ancestors must learn to
treat thoughts as internal states of one another and subsequently to
self-report their own thoughts (1956: SSSS47-59). He
extends the story from the attribution of inner thoughts to one
another to the positing of internal sensory impressions (1956:
SSSS60-62). His view differs from Russell's
position in making the ability to be aware of red sense experiences
depend on the acquisition of a sophisticated web of meaning and
theory; this ability is not "given". Nor, as Russell would
have it, is the universal *red* needed for knowing that s is
red, "easily acquired" (1912 [1997: 101]). Accordingly,
there are no easily acquired color predicates to describe the sense
data that ground knowledge. Sellars asserts that



> 
> 
> instead of coming to have a concept of something because we have
> noticed that sort of thing, to have the ability to notice a sort of
> thing is already to have the concept of that sort of thing, and cannot
> account for it. (1956: SS45)
> 
> 
> 



Sellars claims the advantage that, through this social process of
acquisition, sense impressions lose "absolute privacy" and
become objects of intersubjective knowledge (1956: SS62), and here
he directly departs from
 item 7
 in the Classical Notion. But he has also rejected the acquaintance
view of sense data as immediately present to the mind prior to
learning
 (item 2).
 Sellars's holistic conception that the acquisition of
phenomenal concepts enables the ability to observe and self-report the
presence of sense particulars may offer better reasons for rejecting
item 2 of the Classical Notion than are found in his discussion of the
inconsistent triad.


## 4. Sense Data Recently



In philosophies of mind and perception, sense data fell out of favor
in the 1960s and 1970s. But there have been attempts to restore some
version of the theory. Often, but not always, these efforts sought to
sustain mental sense data as the immediate objects of perception. The
most extensive efforts are by Frank Jackson, in *Perception*
(1977), and Howard Robinson, in a book of the same title (1994).



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



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



The argument relies on a conception of scientific or physical
properties that is widespread in philosophy: that physics has no place
for phenomenal properties or perceptual experience. Accordingly,
physics posits particles and forces, which may cause perceptual
experience in a suitable perceiver, but which themselves are
colorless. Jackson is certainly right that physics today does not
accept something akin to phenomenal color as a fundamental physical
property; rather, it accepts wavelengths and brain processes, and
these possess no such fundamental property. Prior to the evolution of
color-sentient beings, there presumably were no phenomenal experiences
of color, hence none to be counted among physical phenomena.
Nonetheless, physics, especially in the science of optics, accepts the
"subjective *perception* of color" (Nelson 2017:
125) as an object of
 study,[22]
 just as acoustics accepts the experience of pitch as a physical
phenomenon (Weld & Palmer 1925: 322). Arguably, phenomenal
experiences need not be reduced or even reducible to basic physical
processes in order to be counted among physical phenomena and hence
among items to be investigated in the hard sciences, including
physics. Accordingly, color experience needn't be decisively
categorized as "non-physical". Its ultimate ontology is
left open. It might be a mental content as in an intentional theory or
other appearance
 theory.[23]
 The color property might be analyzed relationally, as a surface
property with the power to produce a color experience, which
needn't be a colored sense datum. The metaphysical status of
phenomenal experience need not be decided in order for the conditions
of such experience to be examined in the science of physics.



Robinson (1994) sets forth the most extensive case for sense data in
recent times. His book offers a history of sense-data theory, which he
sees as deriving from Locke, Berkeley, and Hume. Robinson thus
disagrees with the history offered above as regards Berkeley and
Locke. He also specifies a standard conception of sense data (1994:
1-2): that such data possess the sensible qualiteis they appear
to have, and that they are not intrinscally representational (they do
not intrinsically refer beyond themselves; they have no intentional
content). He takes the Argument from Illusion to be the primary
classical argument for sense data in the twentieth century (though not
much invoked before). Most of the other arguments he examines are
related to Variation or Illusion, or to Jackson's argument from
science about color.



Some of Robinson's arguments are new to our discussion, such as
the Time-Lag Argument. It notes that the light reaching our eyes from
stars may be hundreds or thousands of years old and so the star may
have ceased to exist. On the assumption that the object of veridical
perception exists, our perception cannot be of the actual star, and
hence must be of another object. But all perception involves a causal
process that unfolds in time; hence, on the assumption that perception
must be simultaneous with its object, we never perceive the actual
properties of the things that reflect or emit light (1994: ch. 3).
This argument can be extended to a Causal Argument (1994: ch. 6) which
appeals to the fact that we can be made to seem to perceive objects by
non-standard causal processes, such as those that underlie
hallucination. In such cases, we don't perceive an external
object and so there must be another object that we perceive, having
the properties that appear to us (invoking
 item 3 of the Classical Notion).
 These cases could be phenomenally indistinguishable from normal
perception. Hence, arguing that phenomenally indistinguishable items
should be classified as the same type of object (as in Price's
Argument from Illusion, Sec. 2.2.3, above), we should conclude that in
the normal case we also don't directly perceive an external
object.



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



For those who would avoid both mental and non-mental perceptual
intermediaries, the theory of "Naturalized Sense Data"
(NSD) is of interest (Bermudez 2000). It distinguishes between
what is directly perceived and what is immediately perceived. In this
scheme, three-dimensional objects are directly perceived but not
immediately perceived. We immediately perceive the portion of the
surface of the object that is visible at any moment. Because we
immediately perceive a portion of the three-dimensional object, we are
able to make ostensive reference to the three-dimensional object
itself and so, it is claimed, to perceive it directly. NSD assumes
that naive realism holds that we immediately perceive
three-dimensional objects as a whole; NSD therefore rejects naive
realism, since according to NSD we immediately perceive only
surface-portions (2000: 369-372). NSD objects to traditional
sense-data theory as placing an unnecessary intermediary between the
perceiver and ostended objects (2000: 370-371).



There are two problems with this position. First, as Bermudez
observes, if it is to avoid bringing in mentalistic factors, it
requires a physicalist account of color as a surface property, which
has not been forthcoming (2000: 368, n. 14; 373). Second, if a
physical surface-portion provides the immediate object of perception,
then it provides the phenomenal content of our perception. When Moore
endorsed surface-portions as immediate objects, he noted the problem
that if a surface of a round penny (seen obliquely) is the object of
perception, it should appear round. Since he held that it did not, he
felt compelled to say that the sense datum appears elliptical but is
round, thereby sundering the immediacy by bringing a separate
"appearance" into the mix. A defender of NSD might counter
Moore's response by saying that the penny in fact looks round
and slanted, so there is no disparity between physical surface-portion
and phenomenal perception. But then Price's objection arises.
Price allowed for what is now called full shape constancy within a few
feet of the observer: the round penny at a slant would appear round.
But he noted that, beyond a few feet, constancy starts to break
 down.[24]
 This creates a problem for NSD: within the framework set out here, it
would need to recognize "appearances" with content that
differs from the actual structure of a surface-portion. This violates
the intent of NSD, which is to establish an account in which the
phenomenal content of perceptual experience is provided by the
properties of the surface-portion itself, without needing to invoke
appearances in addition to immediately perceived surface-portions.


## 5. Conclusion



The core notion of a sense datum is an immediately given, minimal
perceptual object, consisting in the case of vision of a shaped patch
of color. Often, but not always, this datum was considered to be two
dimensional. It typically was contrasted with the perception of a
three-dimensional material object of a particular kind and displaying
a panoply of further properties. In the classical period (the first
several decades of the twentieth century), sense data were typically
taken to be non-mental and mind-independent, and were often regarded
as representing material objects. In some cases, sense data or their
kin were taken to be elemental constituents in the composition or
bundling of sense particulars. These particulars did not refer to
material objects per se but, when conjoined, composed a perceptual
world (as in Russell 1914a and 1914b). More recently, sense data have
been viewed as mental items (e.g., Jackson 1977), and perhaps as
leading to idealism (Robinson 1994). In a few cases, sense data have
been equated with portions of the surface of a material object (Moore
1918-19: 23-24; Bermudez 2000).



The theory of sense data is a species of a genera of positions in
perception theory, which make a distinction between an impoverished
sense impression and the perception, affirmation, or judgment of a
further entity, usually a material object. Although sharing this
distinction, other species in this genus differ from sense-data theory
in various ways, the most fundamental being that they do not posit an
object having the properties of color and shape as found in conscious
perception
 (item 3).
 Some views render perceived properties as appearances. One type of
appearance theory holds that shape, color, and other perceived
object-properties are intentional contents (intentionalism). Another
type holds that experiences of shapes and colors are manners or modes
of perceiving (adverbialism). Adherents of both types of theory reject
the act-object analysis and so would reject item 8 of the Classical
Notion; and, of course, they would also reject items 1 and 2. In
another vein, some versions of naive realism hold that we
immediately perceive the surface of an object from a point of view and
in three dimensions, and then augment the basic perception to include
conceptualized object content (Noe 2004, Brewer 2011). At
present, these various positions enjoy more favor than do theories
that posit sense data as mental objects or as third things. Indeed,
recent discussions in the philosophy of perception frequently use
sense data as an example of a concept or an entity to be avoided
(e.g., Fish 2010: ch. 2).



Theories that distinguish impoverished sense perception from object
perception and cognition have been challenged by what Firth
(1949-50) called the "percept theory", according to
which immediate visual experience is of a three-dimensional visual
world that manifests object-properties and possibilities for action.
This is the tradition of James, the Gestalt psychologists, Gibson, and
some of the appearance theories (e.g., Smith 2002). Such theories have
grown in favor recently. But sense-data theories have also shown
development. Many sense-data theories recognize the immediate objects
of perception to be three-dimensional. This would permit a penny seen
at a moderate slant to be described as appearing as a circle at a
slant, not an ellipse. Finally, although treatment of color as a
primitive property that literally inheres in sense data (whether such
data are considered to be surface portions, mental objects, or third
things) is not widely favored, it is also true that, metaphysically,
there is no settled home for phenomenally experienced color. The
endeavor to account for the phenomenal characteristics of objects and
their properties is ongoing.