For Aristotle, living things are understood as composites of body and soul. Body, which is matter, is informed by soul, which is form. Matter is also potency, or that out of which something is made, and form is act, or that which makes the matter an actual substance. Following this conceptual correspondence we can say that body, as matter, is potency, and soul, as form, is act. The body and soul composite is merely a specific kind of potency and act composite, namely, the kind proper to living things. Aristotle discusses the different powers of soul and then distinguishes living things according to the powers of soul most proper to them. Plants, for instance, have a nutritive or generative soul, while animals have a sensitive or perceptive soul. Human beings, however, have both the nutritive and sensitive powers, but neither of these are most proper to human beings as such. For humans, the power to understand, commonly referred to as the intellect, is what characterizes soul. There is a difference, though, that divides the human soul from the souls of animals and plants and it is found within Aristotle’s descriptions of each. Whereas the nutritive and sensitive powers require bodily organs in order to be actualized, the human soul cannot have any such organ.
When Aristotle is discussing understanding he compares it to perceiving, both of which somehow consist in the reception of some object. This power of understanding, intellect, is characterized primarily by its ability to receive form. As something receptive, intellect is in potency with respect to all of the things which it is able to understand. While it is understanding something, or after having understood something, the intellect is, in a sense, actually what it understands, it contains within itself the form of the object of its understanding. This presents us with a problem.
Remember that the body and soul division is a specific kind of matter and form division, the latter being found within all material reality and not just in living or organic things. All material substances are, by nature, composites of matter and form. But if the intellect, like matter, is able to receive form, then the intellect, like matter, would actually become the substance in question. For some matter to be in the possession of a certain form, a cat, for instance, is nothing more than actually being this same composite substance. For some matter to be informed by the soul of a cat is just what it is to be a cat. This distinction is at the heart of Aristotle’s ontology, but when applied to the intellect it seems incoherent. If understanding what it is to be a cat is identical with saying that my intellect is in the possession of the form of a cat, then the intellect, if it requires a bodily organ, upon understanding what a cat is, would quite literally become a cat. This is obviously not the case, though. The intellect is able to understand all types of forms and yet is never actually a material substance informed by the understood form in question. The implication, then, is that the intellectual power of the human soul cannot possibly require a bodily organ in order to actualize itself. Hence the intellect must be immaterial.