American Philosophical Association
Pacific Division
Friday, April 2, 1999
Berkeley, California

The Activity of Sensation in Aristotle

by Joseph M. Magee

Miles Burnyeat has recently sparked a debate among Aristotelian scholars over the correct interpretation of Aristotle's teaching on the relationship between sense powers and their organs. He claims that for Aristotle, "... the physical material of which Aristotelian sense-organs are made does not need to undergo any ordinary physical change to become aware of a colour or a smell."(1) Burnyeat implicates Thomas Aquinas in this anti-physiological reading of Aristotle, claiming that Aristotle's several descriptions of sensation refer to the decidedly non-physical activity of perception, Aquinas' 'spiritual' change.(2) Stephen Everson, in deference to the imputation of Aquinas, calls Burnyeat's interpretation a "spiritualist" reading.(3) Everson proposes to defend a literalist interpretation, and like Richard Sorabji, believes that for Aristotle, when an animal perceives, the sense organ undergoes a normal physiological change, as would any inanimate matter of the same sort. Moreover, this physical change is characterized by the organ becoming literally such as the object is, colored in the case of the eye, warm or hard in the case of touch, etc.(4) As we shall see, for both textual and philosophical reasons, Everson's interpretation cannot be the correct one.


In De Anima II. 5, Aristotle distinguishes two kinds of alteration that are applicable to perception. He concludes that "there are two senses of alteration, one a change to a negative condition, and the other a change to a positive state, that is, a realization of its nature" (417b15-17). Everson labels these two senses of alteration "alteration1" and "alteration2" and explains the basic distinction between them: "When something undergoes alteration1, it loses the property it had before the alteration and acquires a 'contrary' property; in alteration2, it simply exercises a capacity it already possesses."(5) Everson believes this chapter supports the literalist interpretation, for he claims that the chapter shows that both kinds of alteration are involved in perception,(6) and appeals to Physics VII. 2 for support. There, Aristotle asserts that in perception there are two sorts of alteration, one of which both the animate and the inanimate are susceptible to, and another of which only the animate, that is the sensate, is capable (244b9-15).(7) According to his account, the material change (alteration1) determines the psychological activity (alteration2), and that this is necessarily implied in Physics VII. 3 (246a4-9).(8) When a man or a house comes to be, the man or house is not altered since it only has just come to exist; its coming to be, however, may have been necessitated by matter undergoing alteration1. "If this is right, then Aristotle commits himself here to the determination of changes at the formal level by alterations at the material level...."(9) Furthermore, to the extent that material changes determine formal changes,(10) this interpretation claims that alterations2 supervene on alterations1; the psychological process supervenes on the material change.

There are compelling reasons against believing that Aristotle even could accept that perceptual activity supervenes on physical alteration. For, the potency which the subject of an alteration1 has at the beginning of the alteration1 is completely actualized by the end, and at the end it is no longer in a state of potency with respect to the same sort of alteration1. What is altered1 is in potency to what it will become, but in so altering1, it thereby loses that potency to be altered1. That is, once it is altered1, it cannot then be altered1 again with respect to the same quality. This is the definition of alteration1. If, however, sense organs were to be altered1 in perception, they would then lose their capacity to be altered again. Such a view of the physical process occurring in sense organs creates insuperable problems when it is connected to perception as an activity.

On the literalist model, the eye, for instance, is made literally red in one instant, and in just one part of its eye-jelly. That part, in that instant, then loses the potency to be affected by red until the affection that is there fades. However, one would expect that, in the next instant, even before the red affection fades, it could be affected by a blue object, turning the formerly red bit of eye-jelly blue. This should hold true because the eye-jelly, even though affected by the red object, is still matter for a living, functioning eye; it should, thus, still have the capacity for sight. If it were true that red-ly affected eye-jelly bits can become blue, then one has abandoned Aristotle's principle that the eye-jelly be transparent in order to be affected by colors. Clearly, then, this alternative is unacceptable.

However, if one denies that the red eye-jelly bit can become blue, on the other hand, and instead claims that the redness of the bit of eye-jelly must fade first, one still encounters problems. Such an account seems contrary to Aristotle's (and Everson's) commitment that perceptual awareness is a continuous activity. For, while looking at the same red wall, one does not ever cease to perceive it. If seeing occurs when the eye-jelly takes on the color of the object seen, however, one would not see the red wall for as long as it took the last moment's affection in the eye-jelly to fade. Perhaps, one could claim that eye-jelly affections fade rather quickly. In this case, while it is true that until the previous affection fades there would be no perceiving, perception would occur intermittently, producing a sort of strobing effect which might go undetected. However, insofar as perception at least involves an activity, it is continuous, and our ability to engage in it is constant, even while already being engaged in it. Thus, the formal cause of perception could not be a single activity if it has to supervene on the strobing of alterations in the organs, since it is at least necessary that what supervenes be simultaneous with what it supervenes on. Supervenience, then, cannot accommodate both standard alterations and activities in an Aristotelian explanation of perception.

The potency which characterizes a sense power in being potentially like its object, then, is a condition of perception that exists throughout the perceptual process. Thus, even while perceiving, the sense organ does not lose its capacity to perceive, and so it does not cease being able to become like its object. Aristotle seems to have had this in mind when he introduces the distinction between alteration properly-so-called and activity by saying that the latter is a preserving (417b3). Fortunately, Aristotle believes that the effect of light and color at least, and presumably by extension the effects of the objects of the other senses, are activities which the physical organ engages in.

Form without Matter.

Those who have interpreted Aristotle's theory of perception as not being a case of ordinary alteration appeal to DA II. 12. It is here that Aristotle claims that all perception is a reception of form without matter, and employs the analogy of a gold signet-ring impressing a block of wax (424a17-24). Aristotle says that the sense receives form without matter, as the wax receives the impression without the iron or gold, but also that it does not do so as gold or bronze. A few lines later, he elaborates somewhat on the meaning of "form without matter" when he considers how the passivity of the senses differ from the way in which insensate things are affected by the same sorts of objects (424a33-b3). Plants do not receive form, apparently without matter; instead they are affected with the matter since they have no mean or principle for the reception of form. Although plants are affected by the objects of touch, i.e. heat and cold, they are affected with matter, and this explains why they do not sense.

The proper interpretation of the idea of the reception of form without matter has been a major point of contention between literalists and spiritualists. Literalists claim that the point of the analogy with the wax block and signet-ring in DA II. 12 is that the gold, i.e. the matter of what makes the impression, is what is left behind, and all that is received is the impression. This impression, however, is a literal and physical impression in the wax. Likewise, the sense organ receives the sensible form of its object, i.e. comes to have literally in itself that sensible form.

But there is good reason to interpret the reception of form without matter physiologically. It means that, for instance, the organ of sight ... takes on the colour of the object seen, without taking on any material particles from the object, such as Empedocles and Democritus had postulated.(11)

Thus, when plants are said to become hot or cold by being affected with the matter, they do so by receiving small particles or vapors of the agent that is making them hot.(12)

Everson defends Sorabji and the literalist interpretation by pointing to what Aristotle says about the physical constitution of plants. He cites DA III. 13, where Aristotle claims that plants, because they are made of earth, do not have a mean for the tangible qualities that belong to the elements other than earth, and this fact explains their insensitivity (435b1-3). Everson argues that Aristotle's reasoning rests on the claim that earth can itself have no qualities other than the cold and dry; these are essential to being earth: " element cannot lose its distinctive qualities without ceasing to be that element...."(13), and cites On Generation and Corruption II. 3 to support this contention. If earth, or something made of earth, appears warm or moist, it is because it has taken into itself some other matter with these qualities.(14)

Unfortunately, this view of how plants take on various sensible qualities is at variance with other texts of Aristotle, texts quite central to Everson's overall argument. First, Physics VII, 2 clearly shows that plants do undergo alteration even in respect of tangible qualities.

Thus we say that a thing is altered by becoming hot or sweet or thick or dry or white; and we make these assertions alike of what is inanimate and what is animate. And further, where animate things are in question, we make them both of the parts that have no power of perception and the senses themselves (244b6-10).

Thus, plants, i.e. animate things without perception, are altered and become, among other things, hot and dry. It appears, then, that the fact that plants are made of earth does not in fact prevent their being literally heated and cooled. Since Aristotle says here that plants become hot by being altered, he cannot think that this happens by their taking on the matter of what heats or cools them.

Moreover, Aristotle explicitly rejects those theories which explain the apparent changes in quality of things by postulating a process by which matter inters into the things that are so affected. In Generation and Corruption, I. 8, Aristotle considers the view of those philosophers who believe that an agent "enters through certain pores, and so the patient suffers action" (324b26). He specifically mentions Empedocles (324b33) and Leucippus and Democritus (325a1) as proponents of this theory. He is extremely critical of these views, however, in spite of the suggestion by Sorabji that he would have advocated such a theory (326b22-24). So even though his predecessors held to the view that things change their sensible characteristics by taking on the matter of an agent of this change, Aristotle explicitly rejects it in GC I. 8. It is extremely unlikely he would have changed his position in the DA when it comes to explaining the heating and cooling of plants.

Another part of GC, II. 3, shows that Aristotle does not believe that the material constitution of plants prevents them from being altered in respect of tangible qualities. Everson appeals to this chapter to show that anything composed purely of earth cannot itself be made hot or cold since earth is essentially cold and dry, and from this concludes that the fact that plants are composed of earth must mean that they are heated and cooled by receiving hot or cold matter. He is right, of course, that the elements do have these qualities essentially (330b4-5). A few lines later, however, Aristotle warns that one should not believe that these 'simple bodies' are to be found in nature in a pure form. "In fact, however, fire and air, and each of the bodies we have mentioned, are not simple, but blended. The 'simple' bodies are indeed similar in nature to them, but not identical with them" (330b20). Thus, the simple body of earth, i.e. the element, is not the earth of our common experience, but similar to it. The earth of common experience, and a fortiori things of experience made of earth, are in fact not simple, but blended. There is, then, no theoretical obstacle to ordinary earth undergoing alteration and receiving the form of heat, say, from an agent, though this would be received into the earth's matter.

Therefore, just on the basis of DA II.12, when the organ receives form without matter, it receives the same form as its object, but not as that form is in the object. Aristotle is explicit that sense is like the wax which receives an impression both without gold and not as gold. The literalists offer no interpretation for this qualification. Aquinas, at least, explains Aristotle's qualification as indicating that the organ does not receive the form in a material way, i.e. not as an alteration.(15) Furthermore, there is no warrant for believing that Aristotle accepted that a plant's being affected with matter means taking on some material vapor from the apparent agent. There are at least three places where he either implicitly or explicitly rejects this.

Opposition to the literalist interpretation of the theory of the reception of form without matter should not be seen as capitulation with the spiritualists, however. Aristotle also explicitly applies the theory to sense organs, so the theory must be meant to identify a physical process, but one that is not an alteration. In DA III.2 (425b22-24), Aristotle claims that it is the sense organ of sight which receives form without matter. Given that the theory of reception of form without matter is not alteration, Aristotle seems to hold the uncommon view that material things can be the subject of alterations2, i.e. activities.

Light and Color as Activity.

This is confirmed in the case of vision, at least. The relationship between light and the perception of color which occurs by means of it indicates that the effect of color on both the medium of sight and the organ is not an alteration, but an activity. Aristotle says that color is not seen without light, "for, as we saw, it is the essence of colour to produce movement in the actually transparent; and the actuality of the transparent is light" (419a9-12). The actuality of light and of color, however, occurs all at once and so could not be an alteration which travels through the transparent medium since the latter processes affect their subjects by stages. To suppose there is such movement "is contrary both to the light of reason, and to observed facts; it would be possible for it to escape our observation in a small intervening space, but that it does so all the way between east and west is too large a claim" (418b21-27). This line of reasoning is repeated in De Sensu, where Aristotle is explicit in his denial that light is a motion. There, he explicitly denies that the medium of sight is made transparent in stages and that the colors of objects reach a mid-point between the object and the perceiver before reaching the perceiver: "light is due to the existence of something, but is not a movement" (446b27-28). Here, Aristotle believes that for other senses the medium is not affected simultaneously, "except in the case of light, for the reason given, and of vision too for the same reason; for light causes vision" (447a11). Both light and vision are not the sorts of processes which progress through space for the same reason, namely, that neither is a motion. Consequently, if color brings about actual vision, it is also not a motion, but is in the medium in the same sense as light is. Therefore, color is in the eye in the same sense as it is in the medium, i.e. as an activity, not an alteration.

Because one sees not only colors, but also sources of illumination such as the sun and fire, these luminous objects of sight give further evidence that vision does not come about from the eye undergoing an alteration. For, light as an activity, not only actualizes the transparent but is also visible (419a23-25). If the activity of fire, for example, allows other things to be seen, Aristotle reasons that when it itself is seen, this will likewise be due to its nature as an activity. Aristotle clearly denies that the transparent medium, and so a fortiori, the transparent in the eye, undergoes an alteration as a result of light; light is the actuality of the actually transparent. Thus, when light itself is an object of vision, it will not be seen through the organ undergoing an alteration since nothing in the nature of light is either the source or subject of an alteration. Rather, the vision of fire, say, occurs when the eyes of the perceiving animal engages in or receives the activity of the light of the fire.


There is, then, ample evidence to reject Everson's contention that, for Aristotle, material alterations in sense organs determine psychological activities, and that the latter supervene on the former. First, activities cannot supervene on alterations since activities are continuous but alterations cannot be. Instead, the sense organs of animals undergo processes which are distinct from alteration which insensate things suffer. In the case of vision, the physical process by which an animal sees is already an activity and matter for the further activity of perception.


1 "Is an Aristotelian Philosophy of Mind Still Credible? (A Draft)," in Nussbaum Rorty, eds. Essays on Aristotle's De Anima, (Oxford: Clarendon Press, 1992), p. 19.


2 Ibid., p. 21.


3 Aristotle on Perception, (Oxford, Oxford University Press, 1997), p. 10.


4 Ibid.


5 Everson, op. cit., p. 92.


6 Ibid., p. 93-94.


7 Ibid., p.136.


8 Sisko also believes that Phys. VII. 3 shows that a denial of material alteration is false. pp. 142-3.


9 Everson, op. cit., p. 271.


10 Ibid., p. 261.


11 Sorabji, "Body and Soul in Aristotle," p. 172.


12 Ibid., p. 217.


13 Everson, op. cit., p. 88.


14 Ibid., pp. 88-9.


15 Commentary on Aristotle's De Anima, Book II, Lecture 24, n. 554.