Sensations are mental states normally caused by the stimulation of sense organs. They are a varied lot. Even though philosophers have traditionally tended to give sensations a unified theoretical treatment, it is not obvious that this is either possible or desirable. Sensations are usually taken to be the paradigmatic bearers of qualitative appearance, but proprioceptive sensations do not seem to have a qualitative character at all. Furthermore, sensations' own nature as mental states is not readily apparent: sensations of hunger or pain appear to us as states of our bodies, and colors and visually presented shapes appear to us to be features of objects before the eyes. The differences among "bodily" and "distance" sensations are marked by common speech, which countenances, for example, "burning sensation," but rarely "visual sensation." The latter must rather be understood as a term of art used principally by philosophers and psychologists, to be justified by its theoretical utility.

Although it is plausible to say of sensations in this extended sense of the term that they are the sources, as well as the ultimate arbiters, of all claims about matters of fact (one form of a doctrine known as empiricism), they, unlike the items of the material world as scientifically understood, are qualitative in character and directly accessible only to the person who has them. This creates two closely related problems that have vexed philosophers from the seventeenth century until the present day. The first problem concerns how each of us may, beginning with our own sensations, obtain knowledge of the material world. The second problem is whether we can reconcile the private and qualitative nature of our sensations with our nature as material beings.

The first problem was widely discussed in the nineteenth and the first part of the twentieth century by philosophers as well as psychologists (Boring 1942), although it has more recently fallen into the background. Here is how it goes. Physics tells us that the world consists of elementary particles and fields. Our sensations inform us that the world contains not only strawberries, but also the colors, tastes, and smells of strawberries. Yet although the strawberry can be understood to be an assemblage of elementary particles, its red color, just like its characteristic smell and taste, is not reducible to physical properties (Hardin 1993). If colors and tastes have no place in the physical world, they must be sensations in our minds, perhaps generated by our brains, though not states of our brains, because our brains are themselves physical objects. But if the strawberry's color is a mental property, must not the shape that it delineates be likewise a mental property? Following this line of thought to its conclusion, Ernst Mach (1897) and some of the earlier positivists held that we are directly aware only of our sensations and can never be entitled to infer the existence of a world of matter that is distinct from those sensations. The world of perceivable physical objects, including our brains, must be understood to be a construction from actual and possible sensations. The unobservable entities of physics are to be regarded as convenient theoretical fictions that enable us economically to represent and predict our sensations.

One alternative to such a radical conclusion is to maintain that we can infer the existence and nature of a material world distinct from our sensations as the best explanation of their characteristics; some of our sensations, such as those of shape, resemble the properties of the objects that cause them, whereas others, such as smell or color, have causes but no counterparts in the external world (Russell 1914). However, it is not clear that an empiricism that uses sensations and the relationships among them as its basis can allow the required conceptions of cause, explanation, or resemblance, all of which require transcending that basis.

Another way to penetrate the epistemic wall of sensations is to deny that they are the direct objects of awareness, and thus the inescapable foundation of our knowledge of matters of fact. James J. GIBSON (1966), for example, effectively argues that our sensory awareness is in many cases best described as picking up unmediated information about the environment (see AFFORDANCES). Although Gibson does suggest how we might have direct access to some features of the physical world, he does not show how experiences that do have a qualitative character could arise simply by having perceptual systems extract information from the environment. Many instances of sensory apprehension seem to involve the awareness of something that cannot be identified with any item in the external world that acts upon the senses (Perkins 1983). How, for example, can a "direct contact" theory account for feeling a PHANTOM LIMB, or seeing a colored afterimage? It seems implausible to assert that in such cases one is either experiencing nothing at all, or else simply misperceiving some item that is outside of one's nervous system.

Our second principal problem is whether sensations can be identical to brain states (see PHYSICALISM), or functions of brain states (see FUNCTIONALISM), and thus find a place in the physical world after all, or whether a scientific psychology could make do without positing sensations (see ELIMINATIVE MATERIALISM). Either of these alternatives is consistent with a materialist view of the world. By contrast, mind-body dualists (see MIND-BODY PROBLEM) suppose that although sensations are caused by brain states, they are in no way reducible to them. This set of issues, nowadays known as the problem of QUALIA, has been of great interest to philosophers and to many psychologists during the second half of the twentieth century. A related question is whether any materialist solution to the problem of qualia must be confronted with an EXPLANATORY GAP. For even if it is in fact true that sensations are identical to brain states or functions, will it ever be possible for us to understand why this must be so? Perhaps our understanding can do no better than record de facto correlations between sensations and brain states. On the other hand, it has been argued that close attention to the character of explanations currently offered by PSYCHOPHYSICS will show that the gulf between sensory qualities and the functional organization of sensory systems is not as wide as has often been thought (Clark 1993). Finally, under what conditions would it make sense to ascribe sensations to machines, or extraterrestrial aliens (Lewis 1980), or closer to home, and perhaps more fruitfully, to other animals (Thompson 1995)?

Additional links

-- C. L. Hardin


Boring, E. G. (1942). Sensation and Perception in the History of Experimental Psychology. New York: Appleton-Century-Crofts.

Clark, A. (1993). Sensory Qualities. Oxford: Clarendon Press.

Gibson, J. J. (1966). The Senses Considered as Perceptual Systems. Boston: Houghton-Mifflin.

Hardin, C. L. (1993). Color for Philosophers. Enlarged ed. Indianapolis: Hackett.

Lewis, D. (1980). Mad pain and martian pain. In N. Block, Ed., Readings in the Philosophy of Psychology, vol. 1. Cambridge, MA: Harvard University Press.

Mach, E. (1897). Contributions to the Analysis of the Sensations. Translated by C. M. Williams. La Salle, IL: Open Court.

Perkins, M. (1983). Sensing the World. Indianapolis, IN: Hackett.

Russell, B. (1914). Our Knowledge of the External World. London: Allen and Unwin.

Thompson, E. (1995). Colour Vision. London and New York: Routledge.

Further Readings

Goodman, N. (1951). The Structure of Appearance. Cambridge MA: Harvard University Press.

Hacker, P. M. S. (1987). Appearance and Reality. Oxford: Blackwell.

Hirst, R. J. (1959). The Problems of Perception. London: Allen and Unwin.

Price, H. H. (1932). Perception. London: Methuen.

Savage, C. W. (1970). The Measurement of Sensation. Berkeley and Los Angeles: University of California Press .