We have covered two, apparently conflicting, lines of thought in Aristotle concerning the nature of the human soul and human intellect:

1. According to the first, one may avoid the inconsistencies of Plato’s account of universals by positing an active intellectual principle, the agent intellect, which is able to form universal representations (the so-called intelligible species) by abstraction from the experience of particulars. (For further details, see again http://faculty.fordham.edu/klima/FILES/form.pdf; http://plato.stanford.edu/entries/universals-medieval/  sections 1-4; http://faculty.fordham.edu/klima/PHRU1000/UNIVERSALS.htm; and the underlined passages in Aquinas’ commentary on Aristotle’s On the Soul in the course reader http://faculty.fordham.edu/klima/phru1000/PHRU1000reader.pdf. For Aquinas’ detailed explanation of abstraction, see pp. 90-93 of Medieval Philosophy.) In possession of the intelligible species, the potential intellect (the power whereby we form simple concepts, combine them into judgments, and reason toward conclusions), informed by these species, is able to combine its universal concepts into universal judgments. Those universal judgments that we know to be true on the basis of our understanding of the natures of the things they are about we know a priori (without experiencing all the things they are true about), and they are the first, intuitive principles of our scientific demonstrations. See again the selection from Aquinas’ Commentary on Aristotle’s Posterior Analytics in the Medieval Philosophy volume (pp. 98-102). Thus, according to Aristotle, we do have a priori knowledge, but the presence of that knowledge in our soul does not have to entail our pre-natal acquaintance with universal Forms as Plato argued, since the agent intellect is able to produce the universal concepts required for a priori knowledge from experience in this life. The soul may start out in this life as a tabula rasa (a clean slate), whence it may come to exist with the birth of a human being. However, this need not mean that the human soul also must die with the death of a human person. In the third book of his On the Soul Aristotle argues that the human intellect, in order to be able to think in a universal manner and in order to be able to think of all material natures, itself must be immaterial. For a brief statement of Aquinas’ understanding of Aristotle’s arguments, see http://faculty.fordham.edu/klima/SMLM/PSMLM1/PSMLM1.pdf, only pages 19-20; the rest of that paper is an advanced discussion of the difficulties involved in Aquinas’ arguments, which we really need not go into in this class. You’ll find Aquinas’ comments on Aristotle’s original argument in the course reader http://faculty.fordham.edu/klima/phru1000/PHRU1000reader.pdf, on p. 9, in paragraph n. 680. However, if these arguments for the immateriality of the intellect are good, then this conclusion seems to be in conflict with the second line of thought we discussed, concerning the Aristotelian idea of the nature of the soul in general.

2. In the second book of his On the Soul, Aristotle argued that the soul, any soul, must be the substantial form of the living body it informs. (For a brief reconstruction of the argument, see again this handout: http://faculty.fordham.edu/klima/medphil/soul.htm; see also pp. 207-209 of Medieval Philosophy, for Aquinas’ explanation. For the distinction between substantial and accidental forms, see again the first two chapters of Aquinas’ On the Principles of Nature, pp.  157-161 in Medieval Philosophy. To check the technical meanings of the terms used see again http://faculty.fordham.edu/klima/medphil/Definitions.htm) Now, if the soul is the substantial form of a living body, then it is certainly not a body itself, but that form which actualizes the matter of that body, making it into an actually existing living body. Indeed, the life of that living body is the very existence of that body, which is also the existence of the soul informing the matter of this body. Thus, the existence of a soul is nothing but its inherence in the matter of the body: for the soul to exist, is for it to inform this matter. In this sense, therefore, the soul is a material form, a form whose existence is its inherence in matter. Soul and body on this conception are united in their very being; they are not two distinct entities united merely in their operation as Plato (and, as you’ll see, centuries later Descartes) thought. (For advanced discussions of the contemporary philosophical importance of this idea, see http://faculty.fordham.edu/klima/FILES/TMCD.pdf and http://faculty.fordham.edu/klima/BodySoul.pdf -- obviously, these readings are optional.) But this yields an apparent conflict with the previous conclusion concerning the immateriality of the human intellect. If the human soul is a material form, how can the human intellect, the power that enables a human being to think, be immaterial? If the intellect is a form that somehow pertains to a human being, and the human being is a body informed by its material substantial form, the human soul, whereas the intellect cannot inhere in any part of the body (since it is immaterial), then how is the soul of this man who thinks related to the intellect, whereby he is supposed to be able to do his thinking?

This conflict was resolved by Aristotle famous Commentator, Averroes, by claiming that the human intellect is a separate, non-inherent (i.e., subsistent) form to which all humans are joined pretty much like terminals to a mainframe computer, by exchanging information. The separate intellect receives the sensory input from humans, does its intellectual processing (so all genuine thinking takes place in the separate intellect) and sends back its output to individual humans. Humans, therefore, only differ from brutes by having access to this separate intellect, but they themselves do not do the thinking that is taking place in the separate intellect. This solution, however, causes more problems than it solves, both philosophically and theologically. (For Averroes and Siger of Brabant, a “Latin Averroist”, on the issue, see chapters 25 and 26 in Medieval Philosophy – these readings are optional.) Aquinas heavily criticized the Averroistic solution on both counts. His own ingenuous solution is that the intellect is not a separate substance, but an individual power of an individual human soul. However, whereas some of the powers of the soul, such as sight or hearing, are accidental forms that inform the living body, i.e., the body informed by the soul, the intellect is an accidental form that informs the soul alone. Thus, the human intellect is an immaterial accidental form of a material substantial form. The intellect does not inform matter, for it only informs a form, the soul. But the soul does inform matter, namely the prime matter of a human body. For the details of Aquinas’ solution, read c. 27 of Medieval Philosophy.

As a further consequence, since the operation of the intellect, namely, thinking, does not take place in any part of the body (it is not the activity of a bodily organ), this operation can continue whether or not the soul is actually united with the body. But then, since the human soul in this way can go on operating (thinking) even when it is not united to the body, it certainly can go on existing as well (since only what exists can operate); therefore, its separation from the body is not its perishing: the human soul, and the intellective human soul alone, is immortal.