In contrast to traditional realism, direct realism holds that physical objects themselves are perceived “directly.” That is, what one immediately perceives is the physical object itself (or a part of it). Thus, there is no problem about inferring the existence of such objects from the contents of one’s perception. Some direct realists, such as Moore and his followers, continued to accept the existence of sense-data, but, unlike traditional realists, they held that, rather than mental entities, sense-data might be physical parts of the surface of the perceived object itself. Other direct realists, such as the perceptual psychologist James J. Gibson (1904–79), rejected sense-data theory altogether, claiming that the surfaces of physical objects are normally directly observed. Thompson Clarke (1928–2012) went beyond Moore in arguing that normally the entire physical object, rather than only its surface, is perceived directly.
All such views have trouble explaining perceptual anomalies. Indeed, it was because of such difficulties that Moore, in his last published paper, “Visual Sense-Data” (1957), abandoned direct realism. He held that because the elliptical sense-datum one perceives when one looks at a round coin cannot be identical with the coin’s circular surface, one cannot be seeing the coin directly. Hence, one cannot have direct knowledge of physical objects.
Although developed in response to the failure of direct realism, the theory of representative realism is in essence an old view; its best-known exponent in modern philosophy was Locke. It is also sometimes called “the scientific theory” because it seems to be supported by findings in optics and physics. Like most forms of realism, representative realism holds that the direct objects of perception are sense-data (or their equivalents). What it adds is a scientifically grounded causal account of the origin of sense-data in the stimulation of sense organs and the operation of the central nervous system. Thus, the theory would explain visual sense-data as follows. Light is reflected from an opaque surface, traverses an intervening space, and, if certain standard conditions are met, strikes the retina, where it activates a series of nerve cells, including the rods and cones, the bipolar cells, and the ganglion cells of the optic nerve, eventually resulting in an event in the brain consisting of the experience of a visual sense-datum—i.e., “seeing.”
Given an appropriate normal causal connection between the original external object and the sense-datum, representative realists assert that the sense-datum will accurately represent the object as it really is. Visual illusion is explained in various ways but usually as the result of some anomaly in the causal chain that gives rise to distortions and other types of aberrant visual phenomena.
Representative realism is thus a theory of indirect perception, because it holds that human observers are directly aware of sense-data and only indirectly aware of the physical objects that cause those data in the brain. The difficulty with representative realism is that since people cannot compare the sense-datum that is directly perceived with the original object, they can never be sure that the former gives an accurate representation of the latter, and, therefore, they cannot know whether the real world corresponds to their perceptions. They are still confined within the circle of appearance after all. It thus seems that neither version of realism satisfactorily solves the problem with which it began. Phenomenalism
In light of the difficulties faced by realist theories of perception, some philosophers, so-called phenomenalists, proposed a completely different way of analyzing the relationship between perception and knowledge. In particular, they rejected the distinction between independently existing physical objects and mind-dependent sense-data. They claimed that either the very notion of independent existence is nonsense—because human beings have no evidence for it—or what is meant by “independent existence” must be understood in such a way as not to go beyond the sort of perceptual evidence human beings do or could have for the existence of such things. In effect, phenomenalists challenged the cogency of the intuitive ideas that the ordinary person supposedly has about independent existence.
All variants of phenomenalism are strongly “verificationist.” That is, they wish to maintain that claims about the purported external world must be capable of verification, or confirmation. That commitment entails that no such claim can assert the existence of, or otherwise make reference to, anything that is beyond the realm of possible perceptual experience.
Thus, phenomenalists have tried to analyze in wholly perceptual terms what it means to say that a particular object—say, a tomato—exists. Any such analysis, they claim, must begin by deciding what sort of an object a tomato is. In their view, a tomato is first of all something that has certain perceptible properties, including a certain size, weight, colour, and shape. If one were to abstract the set of all such properties from the object, however, nothing would be left over—there would be no presumed Lockean “substratum” that supports those properties and that itself is unperceived. Thus, there is no evidence in favour of such an unperceivable feature, and no reference to it is needed in explaining what a tomato or any other so-called physical object is.
To talk about any existent object is thus to talk about a collection of perceivable features localized in a particular portion of space-time. Accordingly, to say that a tomato exists is to describe either a collection of properties that an observer is actually perceiving or a collection that such an observer would perceive under certain specified conditions. To say, for instance, that a tomato exists in the next room is to say that if one went into that room, one would see a familiar reddish shape, one would obtain a certain taste if one bit into it, and one would feel something soft and smooth if one touched it. Thus, to speak about the tomato’s existing unperceived in the next room does not entail that it is unperceivable. In principle, everything that exists is perceivable. Therefore, the notion of existing independently of perception has been misunderstood or mischaracterized by both philosophers and nonphilosophers. Once it is understood that objects are merely sets of properties and that such properties are in principle always perceivable, the notion that there is some sort of unbridgeable gap between people’s perceptions and the objects they perceive is seen to be just a mistake.
In the phenomenalist view, perceptual error is explained in terms of coherence and predictability. To say with truth that one is perceiving a tomato means that one’s present set of perceptual experiences and an unspecified set of future experiences will “cohere” in certain ways. That is, if the object being looked at is a tomato, then one can expect that if one touches, tastes, and smells it, one will experience a recognizable grouping of sensations. If the object in the visual field is hallucinatory, then there will be a lack of coherence between what one touches, tastes, and smells. One might, for example, see a red shape but not be able to touch or taste anything.
The theory is generalized to include what others would touch, see, and hear as well, so that what the realists call “public” will also be defined in terms of the coherence of perceptions. A so-called physical object is public if the perceptions of many persons cohere or agree; otherwise, it is not. That explains why a headache is not a public object. In similar fashion, a so-called physical object will be said to have an independent existence if expectations of future perceptual experiences are borne out. If tomorrow, or the day after, one has perceptual experiences similar to those one had today, then one can say that the object being perceived has an independent existence. The phenomenalist thus attempts, without positing the existence of anything that transcends possible experience, to account for all the facts that the realist wishes to explain.