A distinction is often drawn in philosophy between two types of objects of awareness in perception. First, there are physical objects or substances (such as chairs, books, rocks, and water) and living organisms (animals, plants, and human beings insofar as they are perceptible, that is, their bodies).
A common technical term for all these is material objects. Second, there are data of immediate awareness, which we shall refer to as “sensa” (singular, sensum), such as color patches or shapes, sounds, smells, and tactile feelings.
This distinction is usually fourfold: (a) in status—material objects are external, located in physical space, and “public” (observable by different persons at once), while sensa are private and are usually held to have no external physical existence; (b) in extent— material objects may at one time correspond to several sensa and normally persist throughout the occurrence of many sensa; (c) in directness—the perception of material objects is indirect, that is, it involves inference from or interpretation of sensa that are “given” directly to consciousness; (d) in certainty—one is always certainly aware of sensa but not necessarily so of material objects.
There is no universally accepted term for sensa; sensations and sense data are commonest but indicate a further subdivision. Sensation is customarily used by scientists and psychologists and carries with it the suggestion that sensa are the immediate mental effects of brain activity resulting from the excitation of a sense organ by external stimuli.
It and the less specialized term sense impression may be used interchangeably for the whole experience of awareness of sound, color, and the like, or for any sensum (such as a sound or a color patch) distinguished within it.
The term sense datum (plural, sense data) apparently originated with G. E. Moore but was introduced in print by Bertrand Russell in 1912. It later became particularly associated with the sensedatum theory of Moore, C. D. Broad, and H. H. Price, while Russell developed different views and came to use other terms.
Sense data are not meant to carry any implications of causal theory, and awareness of them is called sensing (the term sense datum is used for the sensum only, not for the whole experience). With the development of the sense-datum theory, controversy arose between those who regarded sense data as objects distinguishable from the act of awareness of them (act/object analysis) and those who denied this and claimed that sensing is really of “sense contents” (adverbial analysis).
But the terminology is generally fluid—for instance, some modern neurologists use the term sense data instead of sensations in causal contexts. Similar concepts are found in earlier writers, though their language is different. John Locke’s “ideas of sense,” George Berkeley’s “ideas” or “sensible qualities,” and David Hume’s “impressions” are all forms of sensa.
It has often been maintained, by philosophers as well as by psychologists, that perceiving consists in the synthesis and interpretation of sensations. But it must be realized that the occurrence of sensations in all perception is only a hypothesis and not an obvious feature of experience. In ordinary language, one may speak of having or feeling sensations of thirst, cold, or pressure and may refer to itches or pains as sensations.
But the technical use of the word sensations involves a considerable extension of meaning, since one then speaks of visual or auditory sensations (that is, colors or sounds), while such locutions have no place in ordinary speech. We do not have green sensations in our eyes, nor do we normally feel or have sounds in our ears.
Admittedly we do have afterimages, spots before the eyes, or ringing in the ears; but these are special cases because, unlike the objects or data of normal perception, the images, spots, or ringing “follows us around” and cannot be avoided by moving the head, closing the eyes, or stopping the ears.
Indeed, in normal perception we are conscious not of colored shapes or of sounds as such but of material objects, or at least of ostensible material objects. Admittedly we may sometimes be aware of sounds, smells, tastes, or feelings of pressure, as distinct from objects or object properties, but it is doubtful how far these can be said to be sensations.
|skin and membranes|
Sounds and smells seem public and external: Two or more people may hear the same sound or smell the same smell and agree on its source; sounds travel, and a smell may fill a room. Tastes are a borderline case—private and in the mouth, yet in a sense external to the skin and membranes—while feelings of pressure or warmth are partly sensations proper and partly seem to be awarenesses of heavy or warm objects. However, colors and colored shapes normally seem quite external, public, and at a distance from us.
Sensations in this technical sense (private mental objects of immediate awareness) are thus mainly hypothetical occurrences. Their postulation can be justified only by its success in explaining the facts of perception, and it rests on two grounds.
First, there is the causal argument—perception of objects depends on and is conditioned by a chain of causal processes; for example, light waves or sound waves stimulate the appropriate sense organ, causing impulses to travel along nerves to the brain and activate the appropriate receiving area.
|G. E. Moore|
Perception cannot, therefore, be direct contact or confrontation with external objects—all immediate awareness must result from the causal process and be an awareness of mental sensations due to brain activity. Since they are thus separated from the external object in time and space, sensations cannot be identified with its properties, though they may resemble them.
Second, there is the psychological argument—many characteristics of perception show that it is not a direct intuitive awareness but involves interpretation of sensations. Thus, error and illusion are really misinterpretations; perception of motion, depth, and distance involves the use of sensory “cues”; and perceptual identification and discrimination are interpretative, not immediate, since they can be improved by learning and experience. (Both these arguments are discussed at greater length under the Perception entry. Here we may simply note some relevant difficulties.)
THE EXTENT OF SENSATIONS. Even if the causal argument forces us to distinguish between external material objects and the immediate objects of awareness caused by brain activity, it does not follow that the latter must be sensations, such as colors or sounds.
|perception of motion|
They may be percepts, that is, mental contents that correspond to whole material objects, though here the psychological argument comes in, suggesting that percepts are the products of interpretation.
Supporters of the theory of sensations, no doubt influenced by discoveries concerning the atomic structure of matter, at one time even claimed that the basic sensations are “atomic,” that they are sensory pointelements, each corresponding to a different nerve cell—a patch of red color would thus be made up of many sensations of red.
This view has now been completely abandoned, largely as the result of the experiments of the Gestalt psychologists, which show that our primary awareness is of organized wholes or figures (Gestalten in German), and not of elements into which these wholes might theoretically be analyzed.
But even though sensations are not now thought of as minute elements that we synthesize, nonatomic sensations (colored patches of a larger size, or patterns of them, as well as sounds, smells, and so on) may still be regarded as data that we interpret in perception.
AWARENESS AND INTERPRETATION OF SENSATIONS. The awareness of sensations or, for that matter, of percepts must itself be explained; the danger is that it will be construed as analogous to perceiving; for example, that seeing objects will be explained as seeing sensations caused by them, which is a circular explanation and can thus lead to an infinite regress—seeing sensations must require seeing further sensations, and so on. (Compare the duplication objection to representative realism in the Realism entry).
It is therefore necessary to maintain that the awareness of sensations or percepts (“having sensations”) is a special kind of direct awareness different from perceiving, an amendment explicitly adopted by the sense-datum theory.
The problems of the psychological argument are (a) that interpretation of anything would commonly be regarded as presupposing consciousness of what is interpreted, and we are normally conscious neither of having sensations (as opposed to perceiving objects) nor of interpreting them; and (b) that the nature of the interpretation of sensations is controversial—a range of theories is possible because it is not introspectable.
The sensationalists (James Mill, J. S. Mill, and others who derived their inspiration from Hume) claimed that perceiving is the association of various sensations. Association is a vague term and was explained as the customary linking of ideas or sensations that are similar, contiguous in space and time, and so on.
F. H. Bradley and other idealists successfully attacked the sensationalist view as inadequate to explain the facts of perception; instead, they claimed that the interpretation is an inference leading to a judgment, supposing that the possibility of error in perception required this. But this overintellectualized perceiving; inferences and judgments are not the only forms of mental activity liable to error.
Arguments for the Introduction of Sense Data
|Arguments for the Introduction of Sense Data|
Since the start of the twentieth century, philosophers have made little use of the concept of sensation in their theories but have instead talked of sense data or sense contents. Though the same things—color patches, sounds, smells, and tastes—have been put forward as examples both of sensations and of sense data, the new terminology marks several changes.
Recognition of the visual depth or stereoscopic qualities of sense data means that one visual sense datum or color patch is usually held to correspond to the whole of the visible part of an ostensible object (so that one may have striped or variegated sense data).
Little detailed attention has been paid to psychological phenomena, except for discussion along traditional lines of error and illusion and their bearing on whether perceiving is a form of judgment. There has also been almost a revulsion from causal arguments, clearly influenced by their tendency to involve one in the notorious difficulties of representative realism.
Instead, a fresh start has been made in the conviction that philosophy has its own distinct contribution to make in the logical and introspective analyses of perception and in the consideration of relevant epistemological issues, that is, of the extent to which perception provides knowledge of external reality. Nevertheless, with some adjustment the new arguments might be supplemented by and in turn supplement the causal and psychological arguments for sensations.
Sense data are defined as whatever is “given” or “directly present” in perceiving; they are the object of sensing, of “direct” or “immediate” or “actual” awareness in perception. The claim that this awareness occurs within perceiving is essential to the analysis.
To most of its exponents it seems a clear fact of our experience as percipients, one revealed by reflective examination. “Direct” is explained by Price (in Perception) as meaning intuitive or “not reached by inference, nor by any other intellectual process.”
|J. R. Smythies|
This formal definition was often supplemented by a kind of ostensive one: Moore, J. R. Smythies, and others gave instructions for looking at an object or scene and picking out the sense datum, such as a colored shape. (Misleadingly, afterimages were sometimes offered as examples of sense data, but their difference from normal perception has already been noted; misleadingly also, some talked of seeing or hearing sense data.)
This definition of sense data naturally raises the question “Why not say that tables, chairs, and other material objects are given or directly seen?” In answering this, these philosophers produce various arguments for distinguishing sense data from material objects.
THE CERTAINTY ARGUMENT. The certainty argument was stressed by Price and by Russell in his search for “hard data,” though it is also found in other sources. Directness or givenness implies certainty—what is given must be limited to what we are absolutely certain of. But in any perceptual situation we cannot be sure that we are aware of any particular material object.
For example, an object that seems to be a tomato may in fact be something quite different—a wax imitation, perhaps, or a reflected patch of light, or a hallucination (that is, not be a material object at all). Yet whatever the illusion may be, there can be no doubt, when we seem to see a tomato, that there is given a red, round, bulgy patch of color, a sense datum.
Another version of this argument is the method of reduced claims; by confronting him with possible sources of error, you force the person concerned to reduce his claim from “I see a tomato” to what he actually and directly sees or, rather, senses: “I see a red, round color patch.”
THE PARTITIVE ARGUMENT. When we observe a tomato or a bell, what we “actually see”—the “objective constituent” of the situation, what is given or sensed—is the colored shape that seems to be its front surface. This is a sense datum.
We assume that the object has other surfaces and has other characteristics, such as causal properties, three-dimensionality, and persistence in time; and if we loosely say that we see a bell, we imply that we are perceiving an object possessing these properties, although we do not directly see or sense them.
This argument, which stresses extent of sense experience rather than certainty, was preferred by Broad and Moore but seems inferior in suggesting that sense data are those parts of an object that we “actually see” on a given occasion—which raises difficulties with respect to illusions.
THE ARGUMENT FROM THE CONTENT OF ILLUSIONS. When a drunkard sees a hallucinatory pink elephant or sees two bottles when only one is present, what is the elephant or second bottle if it is nothing material? The sense-datum theorist answers, “A private object of awareness, a sense datum,” and applies this also to cases of the relativity of perceiving: For example, when a round plate looks elliptical to a person standing at one side, the elliptical appearance cannot be the plate, which is round; it is an elliptical sense datum private to that person.
Indeed, it is argued that at all times we are directly aware only of sense data, since there is no qualitative jump between the cases where one cannot be directly aware of an object, and so must be sensing sense data, and the normal cases where we think we are directly aware of an object.
This gradation or lack of jump is particularly clear in the case of relativity, as when we gradually move from where the plate looks round to where it looks elliptical, but it also applies to many hallucinations where the illusory sensa are integrated with a genuine background.
In short, perceiving a material object involves sensing sense data related or “belonging” to it; when the plate looks round to me and elliptical to you, I am sensing a round sense datum belonging to it and you are sensing an elliptical one.
The Full Sense-datum Theory
|The Full Sense-datum Theory|
The fundamental conception of sense data, as directly given elements of experience, spread far beyond epistemology. Both the atomic facts of the logical atomists and the supposedly incorrigible basic or protocol propositions of the logical positivists had as their prime examples simple statements about sense data (or sensa generally), such as “This is red.” But the conception was also developed into a full theory of perception by consideration of the following topics, even though disagreements led to variant accounts.
THE GENERAL NATURE OF SENSE DATA. The arguments for the introduction of sense data, if valid, show that sense data are given and provide examples of them. Further alleged properties emerge from the discussion of illusions and relativity, namely, that sense data
- are private, each sensed by only one percipient (see argument from the content of illusions);
- are transitory existents, lasting only while they are sensed, so that they are usually claimed to be events rather than things or properties;
- are distinct from the percipient and seem to be external (in contrast with sensations);
- are without causal properties, for sounds (as opposed to sound waves) cannot act on other things, nor can colors or tastes, though the sensing of them may affect a person;
- cannot be other than they appear to be, or the certainty argument is undermined.
Despite wide agreement on most of these points, a considerable divergence of view arose about (3) and (5). Point (3)—that sense data are distinct from the percipient and seem to be external—involves what came to be called the act-object analysis of sensing.
Largely on phenomenological grounds—on how direct experience of color patches, sounds, and such seem to the person concerned—Price and others claim that sense data have distinct existence, that they are objects distinguishable from the act of awareness of them. But some philosophers maintain that the data are only “sense contents” and do not exist apart from the sensing of them any more than does a pain or sensation.
This view is formulated in the so-called adverbial analysis of sensing, namely, that “I sense a red color patch” is properly to be regarded as a statement of how I sense or, to put it in a different way, “red color patch” is an internal accusative of the verb sense, just as “waltz” is an internal accusative of dance in “I danced a waltz.”
There is agreement on point (5)—that sense data cannot appear to be what they are not, for example, sense data cannot appear elliptical when they are round. (Even this is dubious—an apparently pink expanse may, on examination, be found to consist of red dots on a white background.)
But some say that sense data can fail to appear as they are (do not reveal their full properties at first sight); thus, one may see that a colored datum is striped without noting how many or how thick the stripes are. Others deny this, claiming that a closer look results in a fresh sense datum. In fact, the theory cannot deal satisfactorily with the phenomenon of attention.
A thing may look quite different on careful examination from the way it looks at a casual glance, and the difference seems to be a matter of how attentively we look, a matter of changes in our mode of observation.
In line with this evidence, one should say that sense data may reveal their full properties only on a closer examination, but then one is suggesting that sensing may at times be casual and inattentive and is thus undermining the fundamental claim that sensing is certain and incorrigible.
THE RELATION OF SENSING TO PERCEIVING. The distinction between sensing and perceiving is threefold. First, perceiving is the awareness of some material object; except in certain kinds of illusion this awareness is the result of the object in question (or light or sound from it) acting on the percipient’s sense organs.
Sensing is the awareness of private sense data that differ from material objects and do not affect the sense organs. Second, sensing is claimed to be direct, immediate, and incorrigible, a form of knowing.
Owing to illusions, perceiving cannot be this; it is fallible and indirect. Third, the indirectness of perceiving is said to consist in its being mediated by sensing; perceiving involves sensing, contains sensing within it.
Various views are possible about the nature of this mediation of perceiving by sensing, but they are best expressed as theories of perceptual consciousness. The same kind of consciousness of a tomato, for example, seems present in normal perception, when one sees a tomato as a tomato; in an illusion, when what one sees as a tomato is a piece of wax; and in a hallucination, when no corresponding material thing is there.
The kind of consciousness present in these three cases may be called perceptual consciousness and is more conveniently discussed than perceiving, where the implication that there is an object acting on the sense organs complicates the issue.
Some, such as Brand Blanshard, claim that perceptual consciousness consists in sensing a datum and judging or inferring that it belongs to a material object. Price, however, argued that this is too intellectual and does not fit the facts.
We unquestioningly accept or take for granted rather than infer or judge, and therefore he defined perceptual consciousness as sensing a sense datum (or data) and taking for granted that it (or they) belong to a material object. Others have said that we refer the sense datum to a material object, but refer is vague.
Two points of interest arise here. First, philosophers have most often said that we accept or judge that the sense datum belongs to a physical object. This seems obvious only about smells or tastes, and one would on first thought say we assume that the visual sense datum or color patch is the tomato.
There is a reluctance on the part of sense-datum theorists to allow this, presumably because they are influenced by the partitive argument or by their knowledge that ex hypothesi the sense datum cannot possibly be the physical object.
But there seems to be no reason why the ordinary person, whose mental processes are being described, may not mistakenly assume this; one would, for example, say “That patch of white over there on the hill is a sheep” (admittedly, the patch as “public” is hardly a sense datum, but it is the nearest one can get to a sense datum by ordinary examples).
Second, to say that we judge or infer that a sense datum belongs to (or is) a physical object is implausible, for it implies we are conscious of it first as a datum, which is not true to the facts: There is no passage of mind from datum to object, as in inference.
Even to say we subconsciously judge or infer is unsatisfactory, for it seems extravagant to suppose that we constantly do subconsciously what we never do consciously. Price attempts to overcome this by maintaining that to take for granted that A is or belongs to B, one does not need to distinguish them at the time—indeed, the contrary is implied.
Sensing thus comes to be regarded as a sort of sensory core within perceptual consciousness, surrounded, as it were, by the further activity of taking for granted. The two states of mind, sensing the red sense datum and consciousness of the tomato, arise together and simultaneously and can be distinguished only by subsequent analysis.