The Gilson Society
American Maritain Association Meeting
Berkeley, California
October 22, 1999

Aristotle and Aquinas on Proving the Intellect's Immateriality

By Joseph M. Magee, Ph.D.

Saint Thomas Aquinas often argues for the immateriality of the intellect and often employs one argument in particular, which he also attributes to Aristotle. As he explains in his Commentary on the De Anima,

Anything that is in potency with respect to an object, and able to receive it into itself, is, as such, without that object; thus the pupil of the eye, being potential to colors and able to receive them, is itself colorless. ... Since then (the intellect) naturally understands all bodily things, it must be lacking in every bodily nature; just as the sense of sight, being able to know color, lacks all color (Bk. III, Lect. 7, no. 680).

The intellect has no bodily nature since it understands all bodily natures, and it is a condition of knowing a power that it lacks the nature it knows.

Thomas Russman, however, in A Prospectus for the Triumph of Realism, argues that what we know about the nature of perception invalidates the assumptions of Aristotle and Aquinas, and because this argument for the immateriality of the intellect depends on an analogy with sensation, its conclusion is undermined. Russman reasons that just as a pink retina can receive the form of green without any hindrance, so a material intellect can receive the forms of all material things without any hindrance. Since Aristotle and Aquinas turned out to be wrong about their understanding of the senses and used this mistake as a basis for their reasoning about the intellect, there is no reason to accept their conclusion that the intellect is immaterial.

Although Aquinas and Russman attribute this argument to Aristotle, the version found in De Anima 3. 4 differs from it in one crucial respect. Whereas Thomas claims that the intellect is immaterial because it knows all bodies, Aristotle says that mind is 'unmixed' because it thinks all things (panta noei).(429a18-20) This difference, I contend, makes the argument as Aristotle actually formulated it immune from the criticism of Russman, for Aristotle can draw on his own distinction between being subject to a material alteration (taking on a form in a material way) and engaging in the activity of knowing (taking on the form of an object without matter by becoming one with it).

At the beginning of DA 3.4, Aristotle outlines the similarities between aisqhsiV and nouV in order to establish a basis of comparison from which he will conclude that the activity of the latter is not realized in the body.

If thinking is like perceiving, it must be either a process of being acted upon by what is knowable, or something else of a similar kind. This part, then (although impassive) must be receptive of the form of an object, and must be potentially such as its object, although not identical with it: as the sensitive is to the sensible, so must mind be to the knowable (429a12-18).

While he begins by making a conditional claim that they are similar, throughout the whole rest of the chapter, Aristotle assumes that they are similar. Recalling his discussion of sensation in DA 2.5, he says that nouV is a case of being acted upon, yet insofar as it is a cognitive faculty like aisqhsiV, it is not a strict case of this; neither thinking nor sensing is a case of alteration. This point will be crucial for a proper understanding of Aristotle's argument and for saving it from Russman's attacks.

Despite the similarity between nouV and aisqhsiV, Aristotle seeks to show how they differ. The first difference between nouV and aisqhsiV is with respect to their respective ranges, and this difference provides the basis on which to conclude that nouV is not realized in an organ.

It is necessary then, since mind thinks all things, that it should be "unmixed" (amigh), as Anaxagoras says, in order that it may be "in control," that is, that it may know; for anything appearing (paremfainomenon) inwardly hinders and obstructs what is foreign. Hence the mind, too, can have no characteristic except its capacity to receive (429a18-22).

Aristotle asserts that nouV knows all things and apparently accepts the universality of its scope without argument. Given this universality, Aristotle believes this shows that mind is, in the words of Anaxagoras, unmixed (amigh). Aristotle thinks this conclusion is warranted because "anything appearing inwardly hinders and obstructs what is foreign." The argument runs thus:

What appears inwardly to a power hinders and blocks the reception of what is foreign.
NouV knows all things, i.e. no intellect is hindered in its reception.
Therefore, nouV is unmixed.

This argument of DA 3. 4, then, depends on the assumption that cognitive powers that are not separate (i.e. powers which are mixed) have a limited range of objects.

In order to successfully prove his conclusion, Aristotle needs to have a basis on which to relate the inwardly appearing (paremfainmenon) with being mixed. None of Aristotle's other few uses of these terms explicitly delineates what the relation is. There seems to be a connection, however, in the fact that the requirement of sense organs for a suitable material implies that each of the senses is limited to the reception of only one class of sensible object. Thus, if the senses are mixed (i.e., bodily) and they are hindered from receiving the forms of objects other than their proper objects, the link that inward appearance is supposed to provide between being mixed and being hindered should be found in the senses. The form of the argument is such that the principle "whatever has something appear inwardly is hindered and obstructed in receiving something foreign" generates the conclusion that "something which is not hindered is unmixed" only if "all mixed or bodily powers have something appearing inwardly which limits their range of receptivity." Unfortunately, Aristotle does not describe an organ's ability to sense in terms of lacking the inward appearance of something which would block the reception of its object.

In other passages from Aristotle's works, there is an apparent basis for this formalization of the principle which Aquinas articulates in his Commentary. What Aristotle says about the material requirements for certain sense organs does seem to imply that he believes whatever receives the form of an object must lack that form itself. As he says in DA 2.7, "It is the colorless which is receptive of color, as the soundless is of sound. The transparent is colourless, and so is the visible or barely visible, such as the dark is held to be" (418b27-29). Aristotle attributes the suitability of organs for sensation to their having a material that is subject to an activity, but not subject to a material alteration. Here, he claims that it is the colorless and the soundless which are able to serve as the matter in which such activities are realized. The implication, then, is that having a color or sound would prevent each respective material from being able to receive either color or sound. Being colored or having color appear inwardly would prevent some matter from being the subject of the activity of anahylic reception of form. This is also confirmed when Aristotle asserts in DA 2.7 that "that which is to perceive white and black must be actually neither (and similarly with the other senses)" (424a8-11). It seems, then, that Aristotle makes a close connection between something undergoing cognitive reception and its lacking the form so received. Let's call this the formal absence requirement for receptivity.

There is another feature of Aristotle's doctrine which provides a basis for the conclusion that nous is unmixed. Ironically, Aquinas' explanation of Aristotle's doctrine of perception provides the key for understanding this connection between cognitive receptivity and the nature of what is cognitive in his account of what Aristotle means by the reception of form without matter in DA 2.12. Aristotle says in this much disputed passage of DA:

We must understand as true generally of every sense that sense is that which is receptive of sensible forms without matter, just as the wax receives the impression of the signet-ring without the iron or the gold, and receives the impression of the gold or bronze, but not as gold or bronze; so in every case sense is affected by that which has color, or flavor, or sound, but by it, not qua having a particular identity, but qua being such, and in virtue of its form (DA 2.12, 424a17-24).

In his Commentary on the De Anima, Aquinas considers how receiving form without matter is distinctive of cognitive faculties like sensation since in non-perceptual cases of a thing being affected, the patient also receives the form of the agent without its matter.(1) Aquinas explains that although in an ordinary case of being passively affected a thing does receive the form without the agent's matter, the patient still receives form with matter, i.e. within its own matter, since the recipient's matter "becomes, in a way, the same as the material agent, inasmuch as it acquires a material disposition like that which was in the agent."(2)

He argues, then, that the reception of form without matter is in contrast to the patient taking on the quality in the same sense, i.e. in a material sense, as the agent.

Sometimes, however, the recipient receives the form into a mode of existence other than that which the form has in the agent; when, that is, the recipient's material disposition to receive form does not resemble the material disposition in the agent. In these cases the form is taken into the recipient "without matter," the recipient being assimilated to the agent in respect of form and not in respect of matter. And it is thus that a sense receives form without matter, the form having, in the sense, a different mode of being from that which it has in the object sensed. In the latter it has a material mode of being, but in the sense, a cognitional and spiritual mode.(3)

When the form is in the patient in a way other than as that form is in the agent's material disposition, then the patient is assimilated in a way that is not standardly material. The fact that this second way differs from the first, i.e. material, mode, is what warrants calling it "without matter." In this second mode, however, it is still the recipient's material disposition which does not resemble the agent's; thus, the fact that he calls the manner in which form is in the sense a "spiritual" mode should not distract from the fact that even Aquinas believes that this takes place in the organ: "the organ of sense is that in which a power of this sort resides, namely a capacity to receive forms without matter."(4)

He believes, then, that the second mode of receptivity, i.e. coming to have the quality but not according to the agent's disposition, is what Aristotle means to convey by the wax block example.

This reading gives insight into some obscure things which Aristotle had said earlier in the DA. In DA 2.5, after noting that perception is a case of being acted upon, Aristotle concludes that perception is a potency and elaborates on the singular manner in which perception is a potency.

Even the term 'being acted upon' is not used in a single sense, but sometimes it means a kind of destruction by a thing's contrary, and sometimes rather a preservation of that which is potential by something actual which is like it, as potency is related to actuality (417b2-5).

Being a potency is essential to the perceptual process. Like other potencies, perception is a capacity for a certain kind of activity, and, in line with Aristotle's general principles, this capacity is defined in terms of its proper act. For example, the ability to see is defined in terms of the act of seeing, and this, in turn, is defined in terms of its proper object, color. More than being merely a capacity or ability for a certain type of activity, the potency of perception is characterized by the fact that the ability to perceive is not exhausted in being actualized. One's ability to see, for example, and to see the same thing, even when already engaged in an act of seeing, is never lost. The potency characteristic of perception, then, is essential to and distinctive of that activity. Thus, being essentially a potency defines the activity of perception. This means that the actualization of this potency is not of such a sort as to preclude actualization with respect to the same object. Hence, Aristotle says that the potency of perception is a preservation (swthria) (417b4).

The fact that the essential potency of perception is preserved in its operation distinguishes it from ordinary processes involving a transition from potency to act, i.e. alteration. In contrast to the actualization of perceptual potency, the actualization of a potency in ordinary alteration precludes any further alteration with respect to the same quality. Such cases of 'being acted upon' are "a form of destruction of something by its contrary" (417b2-3) for, not only is the previous quality destroyed, but even the ability to be acted upon in the same respect is eliminated insofar as it is destroyed. In ripening and changing from green to red, not only is the green which an apple previously had lost or destroyed, but so is its ability to become red. Being red now, it no longer can become red. The potency an organ has for perception, then, differs from the potency a thing has for ordinary change. For this reason, Aristotle calls the process of perceiving by his technical term of an activity. Perception is either not an alteration or one that should have its own name (417b6-7); it is an activity insofar as it corresponds to the exercise of knowledge (417b18-19).

Perception is the reception of proper sensibles (color, sound, etc.), or sensible form, without matter in the sense that what receives the form comes to have it in a non-matter-like way, as Aquinas admirably explains. Given the misleading and negative connotations of the terms "immaterial" and "spiritual" reception, it seems best to refer to this non-matter-like reception by the term "anahylic reception". Anahylic reception, then, characterizes both the senses and the intellect since, for both of them, they become like their object and receive its form in a manner that is not like ordinary alterations. They each are anahylic receptions since each is essentially a potency, and the potency is not lost in being realized in either the actuality of perception or intellection.

Although perception is an activity, and immaterial in the sense just explained, it is still realized in material things. Aristotle is able to maintain that physical things (sense organs) can be the subjects of anahylic receptions since he believes that other purely physical processes are also activities in his technical sense. Such processes are in fact crucial to his explanation of senses and their organs. In DA 2.7 and De Sensu 6, the change which the transparent medium undergoes as a result of the causal efficacy of light (and by extension also of color) is described as not being a motion and so is not an alteration (446b27-447a11). The effect of light and color is instead an activity, but one that is realized in unequivocally material things, i.e. air and water. Because sight itself is an activity of receiving color, the medium for sight and the matter in which the perceptual ability is realized (i.e. the eye), must be composed of one of these two material substances which are capable of being the subject of the activity of color and light. Likewise, since all the senses require a medium, so all of them are activities realized in material things, i.e. their organs. It is in fact this constraint which the material medium places on sense powers which allows one to draw further implications about the nature of senses and sense organs.

The fact that organs must be made out of matter which can be the subject of an anahylic reception at once allows the possibility of perception and limits the range of each organ. Since each organ needs to be made out of matter which is the subject of an appropriate activity and this sort of matter is the subject of just one activity, each sense is limited to that one sort of activity, the activity of receiving its objects anahylically. Although the eye must be made of something transparent in order to receive anahylically the activity of color, the transparent is receptive of only the activity of color. This entails, then, that the eye can only receive, i.e. know, colors as its proper object. The same principle applies to each of the other senses. The medium of touch, which is in flesh, receives more than one set of contraries because it happens to be anahylically subject to them. It is nevertheless limited to these and no others. It is a consequence of the fact that the senses need to be made out of their appropriate matter, that they are restricted in the range of objects that they each may know. Given that the matter of each is in fact the subject of the activity of only one kind of sensible quality, and this is what constrains the sense to be made of this kind of material, each sense is restricted to knowing only this one kind of quality.

In DA 3.4, Aristotle contrasts mind with sense and by saying that nouV knows all, he explains that the nature of mind is such that it is completely cognitive in the sense that there is no limit to its receptivity of form without matter. Since cognitively receptive things do not undergo material changes insofar as they are receptive (for a nature subject to such material changes prevents cognitive reception), so mind has a nature that is not subject to any material change whatsoever. This feature of nouV is in opposition to sense faculties (e.g., sight) which must be realized in some matter (e.g., water which contains the transparent) that is of such a nature as not to be susceptible to literal changes with respect to its object (i.e., coloration). Sense powers, however, are limited in their range insofar as their matter is subject to only one kind of anahylical reception, e.g. the transparent only receives color. Sense organs are subject to literal and material alterations with respect to other sense qualities of which their matter is not the subject of anahylical reception. Eyes are affected by the tangible qualities: hard, dry and hot. The claim that mind knows all things means that it is materially affected by no sensible quality, and since every material thing is materially affected in some way, mind must not be realized in any material thing, as in an organ.

Thomas Russman, in A Prospectus for the Triumph of Realism, agrees that Aristotle's first argument from DA 3. 4 proceeds according to the analogy with perception outlined above.(5) Russman argues, however, that what we know about the nature of perception invalidates the assumptions which Aristotle makes about sensation, and so the conclusion that the mind acts separately from the body is unwarranted. Russman believes that it is an assumption of Aristotle's that "having a form in such a way as to be something (of that form)" interferes with "having a form in such a way as to know something (of that form)," an assumption which has been seen to be false in the light of contemporary biology and neurophysiology.(6) According to Russman, one can agree that seeing green, for instance, does consist in receiving the form of green, but that this reception is not blocked by the fact that what receives it has a color of its own.

To receive the form of green necessary to see something green is only to be in the sensory/neurological state that corresponds with seeing green. But if this is all that is meant by "receiving the form of green," then already being a certain color does not interfere with or distort it. The colors of the retina, optic nerve, brain, and so on are, as such, irrelevant to what goes on when one sees a green object. They do not distort the green color that one sees.(7)

Thus, Russman reasons, just as the pink retina can receive the form of green without any hindrance or distortion, so a material intellect can receive the forms of all material things without any hindrance or distortion.

Aristotle has said that the intellect must have no material form whatever of its own because this would interfere with reception of the forms needed for knowledge of all material things. He concludes that the intellect must operate independent of the body. But once we properly distinguish between the two ways of "having form," illustrated by color perception, we see that the intellect might very well have its own material form without this form distorting the forms by which it knows. Operation independent of the body is therefore not required to explain how the intellect can be open to the knowledge of all of nature. The Aristotelian argument for residual dualism is completely deflected.(8)

Since contemporary science has discredited the assumptions about sensation upon which Aristotle builds his argument in DA 3. 4, his conclusion that the intellect is unmixed with, and separate from, the body does not follow.

Russman seems to have been unduly influenced by Aquinas in his reading of Aristotle's argument. Both Aquinas and Russman believe that Aristotle argues as follows: Since the intellect receives the forms of all bodies, it must lack the form of any body. They seem to hold this interpretation despite the fact that Aristotle's text merely says that nouV knows all things (panta noei), not that it knows all bodies. Accordingly, Aquinas and Russman believe that the intellect exactly parallels the senses in the relation between receptivity and its own nature: Since the eye receives all colors, it must lack the form of any color. Aristotle himself in texts other than DA 3. 4 also seems to endorse this connection between receiving a forms and not possessing them; the transparent receives color and the soundless sound.(9) It is not, however, necessary that this serve as a basis of his argument that nouV is separate from the body. The fact that he does not say that the intellect receives the forms of all bodies, but instead says that it knows all things, indicates that the analogy with the transparent is not what he bases his argument on.

Aristotle, in fact, makes two different claims with regard to the senses receiving the forms of their proper objects. On the one hand, as has been said and as Aquinas and Russman have made apparent, he says that only matter which lacks a certain class of sensible object is capable of receiving such forms in sensation. For example, the transparent receives color and the soundless receives sound. On the other hand, only that which receives sensible form without matter is capable of sensing. For example, plants and other insensate things do not sense because they do not receive forms in this way, i.e. anahylically.(10) The senses are thereby limited to one class of object. That is, what receives the form of color anahylically, i.e. not as an ordinary alteration, receives only such forms, but it is still subject to receiving other forms materially. It is only the second claim that is crucial to his argument, for only this second claim (and the sense power's implied limitation with regard to objects) which generates the conclusion that nouV is non-bodily when coupled with the claim that nouV knows all things (as opposed to the claim that the intellect receives all bodily forms).

Thus, the discoveries of contemporary science about sense organs and the brain do not necessarily vitiate Aristotle's argument that the mind acts apart from the body. Aristotle can concede Russman's point that pink things (retinas) can receive the forms of colors. He can insist, however, that they do so only by receiving such forms anahylically, i.e. as forms without matter and not as matter. He can also insist that receiving forms in this way entails that they receive only such form (i.e. the retina receives only the forms of colors). This being so, and because they are still bodily organs, Aristotle can insist that they are still subject to being affected by other forms (e.g. heat or hardness) in a material way. Thus, by claiming that nouV receives all forms, Aristotle is claiming that nouV is not at all affected materially, and so it is unmixed; that is, it is in no sense bodily, but separate in a strong sense. As long as retinas and other physiological apparatus of sensation still can be said to undergo anahylic reception of form (and nothing in Russman's argument suggests that they cannot) one is still lead to the conclusion that the intellect is immaterial given that it knows all things.

The physical alterations which sense organs undergo are not the activities of sensing; such activities are still physical (in physical media) but they do not have the characteristics which make a process to be an alteration, i.e. the destruction of one form by its contrary.

"For what appears inwardly hinders and obstructs (the reception of) what is foreign." (paremfainomenon gar kolue kai antifrattei to allotrion) The thing appearing inwardly is the process of undergoing an alteration, and hindering and obstructing what is foreign means being prevented from receiving form without matter. Appearing inwardly, then, does not mean being a part of the constitution of the thing (as Aquinas believes). What is subject to an alteration is prevented from receiving form without matter (anahylic reception) so if there is no prevention of reception of form without matter (it knows all) then it is not subject to any alteration. This then is not really a restatement of the principle the colorless receives color and the soundless sound. It is rather that the uncolorable receives color. Thus, the intellect is unaffectable by anything since it knows everything. (The consequence of this is that pink things as alterable and altered, do not in fact receive the forms of things seen anahylically.)

So being pink does not have much bearing on the anahylic reception of the form of green, but becoming (being altered so as to be) any color cannot be the same as seeing color (i.e. receiving form without matter).


1Thomas Aquinas, Commentary on Aristotle's De Anima, Book II, Lecture 24, n. 551, 171-2.


2Ibid., n. 552.


3Ibid., n. 553.


4Ibid., n. 555.


5Thomas Russman, A Prospectus for the Triumph of Realism (Macon: Mercer University Press, 1987), 24-5.




7Ibid., 26.


8Ibid., 26-7.


9418b27; 424a8-11.


10DA 2. 12, 424a32-b20.