De Anima (DA) Book 1:

At the beginning of his De Anima (On the Soul), a work of physics because it deals with natural extended things that move per se, Aristotle inquires into the nature of the life principle or soul (psyche). Reaching back to the Categories, he raises the question of whether the soul is a substance or one of the 9 accidents. This is a question he answers in Book 2 ch. 1 (2.1). But in our readings on Book 1 he makes it clear that the soul is not just a physical thing and he also makes it clear that he does not conceive of the soul in a Platonic way. Perhaps in a Platonic way one might say that the soul moves the body and can exist separate from the body. If that were the way to proceed, we could say that the soul feels, senses, thinks along the lines of what we say in Plato’s Phaedo.  But Aristotle says that life actions of a human being sometimes necessarily involve the body (feeling, sensing, moving all require a body) but at other times seem not to need a body (thinking). All in all, for Aristotle it seems best not to say the soul feels, senses, moves or thinks but rather we should say that the human being does these things by means of or with the soul. Note that if this is the case perhaps there is no activity of a human being which is separate from the body. Perhaps humans are no different from cats and dogs and other animals and all are just life principles necessarily in bodies. Still, there is that problem of thinking, willing, reasoning which humans do and other animals do not do.  Further, human seem to do these with a kind of freedom from the restrictions of the body.  Later in De Anima 2.2 413b25-28 he writes,

So far, however, nothing is evident about understanding and the potentiality for theoretical study. It  would seem to be a different kind of soul and the only part that can be separated, in the way in which the everlasting can be separated from the perishable.

De Anima Book 2:

With the second book of De Anima Aristotle restarts the discussion of the soul by the consideration of substance (ousia) reminding us of the conclusions of his Physics:  substance (ousia) can be said of indefinite matter, of shape or form which makes matter definite or particular or of the compound or composite of form and matter. In the latter matter functions as potentiality and form functions as the actuality that determines matter to be a particular thing. Natural living bodies are all compound substances of the third type.

Now actualities are of two kinds: the first actuality is living, the second is actually doing something in an active way such as being awake (second actuality) versus sleeping (first actuality as living but not actively using other powers). The definition of soul, then, is “the first actuality of a natural living body that is potentially alive.” 412a28 But since complex living bodies have multiple organs as  do plants, he gives “an account common to every sort of soul” and says “we will say that the soul is the first actuality of a natural organic body.” 412b5 He later adds after further analysis,

Being awake, then, is <a second> actuality, corresponding to cutting or seeing. The soul is <a first> actuality corresponding to <the faculty of> sight and to the potentiality of the instrument <to cut>; and the body is potentially this.  And as an eye is the pupil plus sight, so an animal is soul plus body.

It is clear, then, that the soul is not separable from the body. At least. some parts of it are not, if it is divisible into parts; for the actuality of some <parts of the soul> is <the actuality> of the parts <of the body> themselves. Still, some <parts of the soul> might well not be actualities of any body and might therefore be separable. Moreover, it is still unclear whether the soul is the actuality of the body the way a sailor is of a ship.

Let this, then, be our outline definition and sketch of the soul.

DA 412b29-413a4

For Aristotle the soul is the final, formal and moving cause of the body and the body is the material cause of the human being. 

Later Aristotle goes on further to discuss the powers or potentialities and activities of what has soul in the chapters that follow which I will not summarize here. But note that in 2.4 he rethinks the definition in terms of function and indicates that what has soul has the powers of growth and decay, nutrition and reproduction, sensation, locomotion and it also seems that thinking is also an activity indicative of life or the life principle called soul. There is a problem here. Aristotle wants to say that what has life also has at least the powers of growth and decay, nutrition and reproduction.  But he also knows that the heavens do not have these powers yet seem to be alive. And also god seems to have the power of thought but without having these basic powers of life, basic powers that seem that they should belong to any living thing. Is god who thinks alive or not? His solution is to circumvent the problem by saying that anything that has any of these powers should be said to be alive and have soul. But does that solve the problem? Why? Why not?

Perception for Aristotle is the actualization of a potentiality in the organ of sense. Sense perception, then, is not a body but the actualization of a power in a body. It is the realization or fulfillment of a power. In the case of may senses the organ can only work within a range as for hearing and can be damaged by hearing something too loud. When the organ is damaged, the logos or range of balance in it can be diminished, damaged or lost. Sensation, then, requires an organ and is a harmonic balance in that organ such that hearing is a range of disturbance of that balance.  This is discussed further in 2.12 where he says sense perception is like the reception of a stamp on a wax tablet.

Wax, for instance, receives the design on a signet-ring without the iron or gold; it acquires the design in the gold or bronze, but not insofar as the design is gold or bronze. Similarly, each sense is affected by the thing that has color or flavor or sound, but not insofar as it is said to be that thing <for instance, a horse> but insofar as it has a given quality <for instance, color> and in accordance with the form <of the sense>. DA 2.12 424a19-24

De Anima Book 3

Here Aristotle discusses imagination (3.3) and then two aspects of intellectual understanding. In 3.4 he explains what knowledge is and how it comes about in a human being. In 3.5 which is one of the most difficult chapters in all of Aristotle’s works, he explains that we must distinguish potentiality and actuality among the powers related to the human ability to have knowledge or intellectual understanding.

In class on Tuesday 12 April we will read very closely and intensively De Anima 3.4 and 3.5. You should read these three pages several times and think deeply about them BEFORE CLASS. I want to have a philosophical discussion of what Aristotle says there without lecturing at you. So, while you should have read all the other assigned readings in Aristotle’s De Anima, for Tuesday make sure above all that you have read De Anima 3.4 and 3.5 two or three times and have thought deeply about those chapters. As the Boy Scouts say in their motto, Be Prepared!

Addendum

Aristotle, De Anima 3.8 from http://classics.mit.edu/Aristotle/soul.3.iii.html

Let us now summarize our results about soul, and repeat that the soul is in a way all existing things; for existing things are either sensible or thinkable, and knowledge is in a way what is knowable, and sensation is in a way what is sensible: in what way we must inquire.

Knowledge and sensation are divided to correspond with the realities, potential knowledge and sensation answering to potentialities, actual knowledge and sensation to actualities. Within the soul the faculties of knowledge and sensation are potentially these objects, the one what is knowable, the other what is sensible. They must be either the things themselves or their forms. The former alternative is of course impossible: it is not the stone which is present in the soul but its form.

It follows that the soul is analogous to the hand; for as the hand is a tool of tools, so the mind is the form of forms and sense the form of sensible things.

Since according to common agreement there is nothing outside and separate in existence from sensible spatial magnitudes, the objects of thought are in the sensible forms, viz. both the abstract objects and all the states and affections of sensible things. Hence (1) no one can learn or understand anything in the absence of sense, and (when the mind is actively aware of anything it is necessarily aware of it along with an image; for images are like sensuous contents except in that they contain no matter.

Imagination is different from assertion and denial; for what is true or false involves a synthesis of concepts. In what will the primary concepts differ from images? Must we not say that neither these nor even our other concepts are images, though they necessarily involve them?