One of the most significant concepts of Descartes’s philosophy is the distinction between soul and body. This concept stipulates the basic theoretical framework into which more data is to be integrated. Of central significance for understanding that soul and body unities are understanding of the soul and the distinctions between act and strength, matter and form, and proper and common sensible. Descartes underlines that all creatures possess the faculty of sensation (one of the abilities of the sensitive soul), and this operates through the body organs.
Descartes describes that the figures appear on the surface of the pineal gland by the departing animal spirits “must be taken for ideas, that is to say, for the forms or images which the reasonable soul considers immediately when united to this machine, she imagines or senses some object” (Descartes cited Grene 33).
For Descartes, mind and the imagination, as faculties of the soul, are reliant upon the intactness and unity of the physical body for their ability to exercise their functions, but also he refrains from identifying either of them with a specific physical organ. There are some indications in the physiological statute that he took the heart to be the seat of the mind. Even if one does concede that the heart is the seat of common sense, as some commentators suggest, it would still not be identical with the heart considered as a physical organ but would be a power or actuality of the heart (Cottingham 65).
There is some controversy over whether Descartes should be interpreted as saying that human beings possess three souls (nutritive, sensitive, and rational) or only one, but the view that we possess only one (with the rational soul bringing with it the powers of the two lower souls, which were then called “faculties” of the soul) was defended by Aquinas. Descartes would have been taught, Aristotle. Despite the controversies about the relationship between the rational soul and its sensitive and nutritive faculties, however, the Aristotelians were still united in their belief that the soul is the form of the body and not a distinct substance, and they regarded sensation as one of the powers of the soul, albeit one that the soul necessarily exercises in and through corporeal organs (Grene 13).
Following Descartes, unlike plants, animals have sense organs that can receive the forms of objects without their matter. This is possible both because of the physical constitution of the sense organ and the fact that it is part of a living being with a sensitive soul; the soul can exercise its faculties only inappropriately organized matter. In the case of senses that involve an external medium, the material organ must be composed primarily of the element that serves as a medium for that quality and must be in some sense neutral with regard to the extremes of the qualities it discerns (Grene 19). Descartes writes:
Finally, it is known that all these movements of the muscles, and likewise all sensations, depend on the nerves, which are like little threads or tubes coming from the brain and containing, like the brain itself, a certain very fine air or wind which is called the ‘animal spirits” (Descartes cited Grene 88).
This can take the form of a complete absence of the quality it discerns (as the water in the eye is colorless), or it can take the form of having the quality but being in the middle range between extremes. Thus the flesh cannot be too hot or too cold, too hard or too soft. Although a physical change occurs in the sense organ when it takes on the form of the sense object, the sensation is not to be understood as identical with that change (Cottingham 46).
The same is true of the faculty of imagination that, while having a physical basis in the movements conveyed inward from the senses, is nonetheless not merely reduced to some spatially localized body part. Descartes attempts a radical simplification of the scholastic theory of perception. He declines to give a complete account of what the mind and body are, or of how the body is informed by the soul, since he wishes to “make no assertions on matters which are apt to give rise to controversy, without first setting out the reasons which led me to make them.” Indeed, one thing that is striking about his brief discussion of perception in Rule XII is the frequent occurrence of disclaimers of this sort.
These indicate that despite the superficial similarity between his view and those of the scholastics, Descartes anticipated opposition from them, which he is attempting to head off (Grene 87). In addition, he is already trying to move the reader in the direction of thinking in a quantitative method as opposed to the way the Aristotelians thought about physics, and he continues to do this throughout his discussion of perception.
Although any theory of perception must account for the fact that a malfunctioning body can cause perceptual errors, Descartes’ sharp dualism makes this problem particularly intransigent, especially since he has eliminated the “forms” that served as a kind of bridge between intellect and world and left the mind confronting figures traced in the imagination — and these are clearly something physical. Thus, the danger that our knowledge of the world will come to be regarded as indirect is particularly serious for Descartes (Cottingham 97).
The problem as Descartes understands it is how to transmit the retinal images to the brain since he believes that the soul has its seat deep within the brain. This belief, in turn, is based on such evidence as to the fact that damage or disease of the brain impedes sensation even though the rest of the body is intact, and damage to the nerves going from, say, the foot to the brain, prevents our having any sensations from the foot. Descartes’ main argument for believing that a merging of the two images must take place on a physiological level is that he believes it is necessary to account for the fact that we see one object although there are two retinal images conveyed inward to the brain.
What sensation we experience depends on which nerves are stimulated (those from the ears make us hear sounds, those from the tongue savors, etc.) and upon the good pleasure of God who connected various color sensations with particular sorts of motions in the brain. Descartes’ thinking about which model to employ is still in flux in the optical writings, and as a result, the distinction between what we perceive by sense and what involves judgment by the mind is not sharply drawn (Grene 43).
It should also be noted here that here the extent that Descartes’ account of situation perception relies on the soul’s ability to be present in the hands or eyes and to direct its attention out from them in straight lines, it is in tension with his position in the preceding discourse (Grene 77). In the fourth part of the discourse of the Dioptrics, he states that the soul senses inasmuch as it is present in the brain where it exercises the faculty of common sense and that while remaining in the brain it can, by means of the nerves, receive impressions from external objects. The notion of the soul using the body, however, harks back to the older form of philosophical dualism in which the soul is radically non-spatial and therefore does not fit well with Descartes’ form of dualism, in which the soul is localized at the pineal gland (Copleston 33).
Descartes supposes that people do not simply have motions in mind to go on, as it were, since God has conjoined souls and bodies in such a way that these motions give us an awareness of the position of various parts of our bodies. This is an institution of nature; we do not need to retrace the nerves to our extremities or engage in any sort of geometrical calculations — the awareness is simply given to us. It forms the foundation of our ability to know the direction in which objects lie relative to the body.
“Having thus considered all the functions belonging solely to the body, it is easy to recognize that there is nothing in us which we must attribute to our soul except our thoughts” (Descartes cited Grene 33).
Since situation perception is a component of the mechanisms described as natural geometry, this enables Descartes to assimilate vision to touch in a way that downplays the explicitly mathematical elements in his account of distance perception. Another possible reason for Descartes not drawing a sharp distinction between seeing and judging might be that drawing this distinction is not particularly central to his own purposes in the optical writings (Grene 76).
So long as he is able to explain vision without recourse to the scholastic conceptual apparatus and to make suggestions for the improvement of vision based on his theory, he regards his theory as successful. And if he finds it necessary to draw the distinction between seeing and judging differently in some other context (as he does in his Sixth Replies), he believes he can do so without thereby radically altering his basic theory of vision in the Dioptrics (Copleston 31).
Descartes’ omission of situation perception is symptomatic of a deeper change — namely, his complete omission of our awareness of the spatiality of our own body as a significant factor in visual-spatial perception. With it goes the assimilation of vision to touch that plays such an important role in Descartes’ earlier discussion of distance and situation perception. In line with this downplaying of the role of the body, those means of distance perception that do not involve a clear, rational calculation (e.g., physical changes) drop out, signaling progress in the direction of involving the homunculus model in all sensitivity.
It is natural to read Descartes as believing that what is given on the second level of soul is, in the case of interaction, a pattern of isomorphic relations with the retinal image, but he does not explicitly say this. It is, though, hard to see what else it could be, since colors are perceived as spread out in space and not as all-pervasive like odors, and based on his physiological theory people would naturally expect them to be arranged as they are in the retinal image. Descartes does, though, do better when it comes to clarifying the problem of whether the corrective judgments are cognizant or not (Grene 81).
In sum, Descartes sees the soul and body as union and dependent parts which comprise the human. Descartes makes advances in vision, and his theory is of great significance for other philosophies and to the genesis of early modern theories of perception. Descartes can be criticized from two directions. Descartes’ understanding of body and soul suffers from some subjectivity and tensions.
Explaining soul perception Descartes puts a strain on his theory, which he tries to explain either by utilizing a kind of inner object that threatens to entangle him in an infinite retreat or by retreating in various methods from his official dualism. For him, the mind interacts with the body only at the pineal gland. Descartes underlines that the mind spreads throughout the body, thus enabling the soul to be present in the eyes or hands to know the places where they are and to direct its attention out from these places.
Works Cited
Cottingham John. Descartes. Oxford: Basil Blackwell, 1986.
Copleston Frederick. A History of Philosophy. Book 2, vol. 4. New York: Doubleday, 1985.
Grene Marjorie. Descartes. Minneapolis: University of Minnesota Press, 2002.