An area of both classical and contemporary dispute in interpreting Aristotle’s theory of perception is with regard to how perception involves the sense-organ taking on the sensible form of the perceived object without its matter.[1] In this paper I will consider St. Thomas’ interpretation of Aristotle on this matter with regard particularly to the sense of sight.


It is a noteworthy fact that Aristotle did not see the study of the soul as strictly speaking an independent science.[2] Aquinas was certainly cognizant of this.[3] The study of the nature of the soul is allocated to natural philosophy (phusikoς), which provides the account for the natural principles of the living.[4] Aquinas takes the methods given when natural philosophy is accounted for in principle and applies them to the study of the soul.

Natural philosophy’s common method of beginning from the confused universal and moving to the particular universal is that which explains both the order of Aristotle’s general exposition of sensation, and specifically the sense of sight.[5] It is through Aquinas’ particularly astute exposition of this procedure that we arrive at an account of how the case of sight involves the sense-organ taking on the sensible form of the perceived object without its matter.

Most generally, the powers of the soul are defined by their activities, and these are likewise defined by their objects. Thus objects are the first things which ought to be considered in the study of the soul.[6] So also Aquinas recognizes Aristotle to begin with the sense-objects when explaining the senses because “objects are prior to faculties.”[7]

The sense objects which are most important for Aquinas’s exposition are the special sense objects: “The very essence and definition of each sense consists in its being naturally fitted to be affected by some such object.”[8] These are the objects which are directly and properly perceived (proprie per se sensibilia). Related to this, Aquinas notes that although we cannot but use these terms, the senses are not properly said to be ‘acted on’ by these objects, but are acted on secundum modum of a sensible. Hence the faculty becomes like the object in sensing, but only sensitively and not substantially.[9] This is a key point Aquinas applies to all the senses. At the same time, Aquinas asserts the sense faculties are the acts of bodily organs and, following the principle “whatever is received is received in the mode of the receiver,” the sense faculties receive a similitude of the thing sensed in a bodily and material way.[10] Reconciling these seemingly incongruous principles, namely that the senses sense in a material and bodily way, and also that they are not acted on in the strict sense of the way body acts on body is at the heart of Aquinas’s interpretation of how the sense organ taking on the sensible form of the perceived object without its matter. Showing how Aquinas proposes to resolve this incongruity for all of sensation is outside the purpose of this paper. But I will next examine Aquinas’ subtle exposition of Aristotle which resolves this incongruity in the sense of sight. This exposition, it seems to me, provides the key to explaining how sight involves the eye taking on the sensible form of the perceived object without its matter.

Aquinas, following Aristotle,[11] notes first that “the proper sense-object being that which each sense perceives of itself exclusively, the sense object of which the special recipient is sight is the visible.”[12] The ‘visible’ is simply a broad term used to include both colour and the translucent visible object without a name for things such as glow-worms and certain fungi. This is straightforward enough.

The difficulty is how the visible object (most importantly colour) [13] acts on the sense of sight. Aquinas sees Aristotle’s foundation for the answer to this particular problem in his first showing what colour has to do with visibility, and secondly in settling what is required for colour to be seen.[14] With regard to the first, Aquinas notes along with Aristotle that the object of sight, namely colour, as such is essentially visible.[15] However, colour is essentially visible not in the sense that it is always visible, but insofar that it possesses the intrinsic qualities to be visible. This is clear from the fact that colour manifest itself only in the right conditions. These right conditions are what Aristotle calls the “transparent” and light.[16] Aquinas’ explanation of these conditions settles the second task of explaining what is required for colour to be seen.

The “transparent” are any substances under the ratio of possessing the quality of being receptive to colour, such as air, water, and solids such as glass. These substances are receptive of colour “from without” such that they render themselves visible vis-à-vis the concomitant colour, and hence are not visible per se. Because the presence of light renders this quality actual, light is defined as the act of the transparent as such; that is, it is light which makes this quality of transparency to be in these things.[17] For example, air is a medium between some coloured thing and my eyes, and when light is present in the air, I can see the coloured thing.[18] From this it is clear that the nature of light must be centrally related to how the eye in seeing takes on the colour of the perceived object without its matter. Appropriately Aquinas next carefully examines the nature of light.

Aquinas thus states that it is clear enough that light cannot be a body, because it would follow that the body of light would coexist in the body of the transparent-- an impossibility.[19] Aquinas points out that on the other hand, “it is impossible that any spiritual nature should fall within the apprehension of the senses, whose power, being embodied, cannot acquire knowledge of any but bodily things.”[20] In other words, the light which is perceived by sense is not spiritual [21](natura spiritualis).[22] This seems to open Aquinas to an obvious difficulty: If light is neither a body nor spiritual, then what is it? His reply runs as follows. Though strictly speaking light is not spiritual (in the sense it does not need matter), it is the spiritual or formal quality of light which brings the transparent into act and consequently is the cause of seeing. Seeing is the reception of the form of colour through light; to bring the transparency into act is the formal quality of light.[23] This is to say that it is primarily light as a form which disposes the act of colour to be visible through transparency.[24] But just as light is a formal quality, there must be some matter in which the light exists, albeit the most subtle form of matter, the “most spiritual.”[25] This matter acts on the eye, but not precisely so as to make the eye to see, but merely to act on the matter of the eye which is seeing due to another principle. What is this matter? It is nothing other than the “transparent body.”[26]

This is amendable with modern physiology and optometry which has explained how light waves act on the eye. Light-waves can be understood as the transparent formed by light, i.e. the matter simultaneous with but nevertheless different than the formal of aspect seeing which is the reception of colour through the transparent.[27] This two-fold way the sense is acted upon can be made clearer by taking the easier parallel case of touch. In the act of touching, the object touched makes an indentation on the skin. This indentation is precisely how the sensible matter is materially affected when sensing, but the indentation as such made by the object is clearly not the sensation.[28]

Admittedly, Aquinas seems a bit hesitant in his interpretation here. On the one hand he calls sight the “most spiritual” and “maximum spiritually,” which implies a comparative order with the other senses, such that sight has the same material/formal aspects as the lower senses.[29] On the other hand, he also states in the same place that light is “only a spiritual change,” implying sight is sui generis apart from the other senses.[30] However, in light of Aquinas’ universal statements made about sensation being bodily and his direct arguments against light being strictly speaking spiritual, it seems more reasonable to take him to be formally restating the point that the proper ratio when considering the cause of sight is the formal quality of colour through light and not its material aspect of the transparency.[31] Light acts on transparency, thus enabling the colour to be such that of itself it has its power to impress its likeness on the medium, and also then on the eye. This principled explanation seems best to answer how the sense organ of sight takes on the sensible form of the perceived object without its matter: the matter the eye is affect by is the transparency, which is accidental to sight. It is the colour which is able to impress its formal quality on the eye through the transparency in act, and this is vision.

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[1] Considered by Aristotle in De Anima (hereafter DA) 416b32-418a6. Aristotle, De Anima, Transl. H.G. Apostle (Iowa: The Peripatetic Press 1981).

[2] As Amelie Oksenberg Rorty points out in “De Anima: Its Agenda and Recent Interpreters.” Essays on Aristotle's De Anima, eds. M. Nussbaum and A.O. Rorty (USA: Oxford University Press 1995).

[3] The order of the sciences is the first thing he mentions at the beginning of his commentary on the DA.

[4] I.e. the physical study is concerned with the principles of the living (arche ton zion), c.f. DA 412a7, 403a27-28. If there are any powers of the soul which are able to exist outside the provenance of natural philosophy they then are the concern of the metaphysician, c.f. DA 403b15.

[5] C.f. Physics 184a10-15. Aristotle, Physics, Transl. H.G. Apostle (Iowa: The Peripatetic Press 1980).

[6] DA 415a14-416b31.

[7] Book II Lectio XIII, para. 383. Aquinas, Commentary on Aristotle’s De Anima (hereafter CDA), Transl. By K. Foster and S. Humphries (Indiana: Dumb Ox Books 1994).

[8] Ibid. Book II Lectio XIII, para 387. Thus I am putting aside Aquinas’ explanation of the sensus communis sense objects which are part of sense but not in the same essential way.

[9] Ibid. Book II Lectio XIII, para 381. Aquinas notes that because the pre-Socratics failed to see this distinction they thought the sense-faculties were composed of the same elements as their objects.

[10] CDA Book II Lectio XII, para. 377.

[11] DA 418127-29.

[12] CDA Book II Lectio XI, para. 399.

[13] Aquinas claims the unnamed glowing object of sight is only mentioned by Aristotle incidentally, and that such creature are the way they are due to a certain weakness of not being able to actualize their proper colors seen during the day. The basic point seems fair enough, namely that the glowing object of sight is a certain exception, and thus I will set aside a consideration of it here. At any rate, it turns out that it is in virtue of the normative relationship of colour to seeing that this exception is explained. See CDA Book II Lectio XV, para. 430.

[14] In DA 418a29-418b4 and DA 418b4-9 respectively; c.f. CDA Book II Lectio XIV, para. 400.

[15] DA 418a31 and CDA Book II Lectio XIV, para. 400.

[16] DA 418b1-3.

[17] Again, “it is evident that neither air…nor anything of the sort is actually transparent unless it is luminous.” CDA Book II Lectio XIV, para. 405.

[18] Conversely the transparent in potency is darkness.

[19] DA 418b17. Aristotle gives further arguments for this position in DA 418b20-26, but the force of the above argument is strong enough to prove his point.

[20] CDA Book II Lectio XIV, para. 415-416. This further point is interestingly left unstated in Aristotle, but seems indirectly affirmed in DA 419a25-31.

[21] Ibid. Book II Lectio XIV, para. 416.

[22] Ibid. Book II Lectio XIV, para. 424.

[23] Thus Aquinas places light in the third species of quality, affective quality. C.f. Book II Lectio XIV, para. 420.

[24] This can conversely be said the way Aristotle states it: “to be a color is to be that which causes motion in that which is transparent when in activity.” DA 419a10-11.

[25] “Sed in immutatione visus est sola immutatio spiritualis: unde patet, quod visus inter omnes sensus est spiritualior, et post hunc auditus.” Ibid. Book II Lectio XIV, para. 418.

[26] Ibid. Book II Lectio XIV, para. 421.

[27] This is clearly Aristotle’s conclusion in DA 419a7-15.

[28] As drawn out by Aquinas in Ibid. Book II Lectio XIV, para. 418.

[29] See footnote 80; Ibid. Book II Lectio XIV, para. 418.

[30] Certain contemporary philosophers have focused too much on this apparent hesitancy in Aquinas and have gone on to argue that for Aquinas sensation must be wholly spiritual and in no way material. See for example, Burnyeat, Myles, “Is an Aristotelian Philosophy of Mind Still Credible?,” Essays on Aristotle's De Anima, eds. M. Nussbaum and A.O. Rorty (USA: Oxford University Press 1995). According to Burnyeat, Aristotle thinks the sense-organ taking on a sensible form is nothing more or less than the awareness of that form. He asserts that there is “no physiological process which stands to a perceiver’s awareness of colour or smell as matter to form.” (pg. 15) Because this is open to obvious refutation by modern biology, Burnyeat ultimately proposes to “junk” Aristotle. (pg. 26)

[31] Likewise one would read Aristotle’s arguments against Empedocles: light as a vulgar material body would indeed be “extravagantly postulated” to move instantly across the horizon. Rather, with light’s more formal nature it would seem to move very, very quickly: 186,282.397 miles per second, to be exact.