In our modern era a book titled “Of the Soul” could be nothing but a theological work and, without thinking, I assumed that that’s what I would be reading when I started Aristotle’s De Anima. I quickly realized, though, that this work is primarily concerned with biology. Unlike Plato’s Phaedo, for example, which is also largely concerned with the soul, there is no consideration of how an individual’s moral conduct (or lack thereof) affects there soul, or how the soul’s condition relates to virtue. There are only passing references to the afterlife: Aristotle argues that a particular soul is shaped or fitted to a particular creature, and can not be transferred to another, thus refuting the Pythagorean notion of transmigration. He doubts whether a soul could can survive outside the body at all. One gets the impression, however, that both points are secondary to his main inquiry.
To Aristotle, the “soul” is nothing more or less than the essence which makes a creature alive. In a characteristically teleological Aristotelian formulation the soul is the “final cause” of the body or, conversely, the purpose of the body is to be an “organ of the soul”.
The book begins with a lengthy literature review in which Aristotle mentions previous work on the soul by Plato and many other philosophers, mostly dismissing their theories on the basis of logical errors or because they are so abstract as to be ridiculous (e.g. those theories that attempt to prove that the soul is number or geometrical figure). The remaining two thirds of the book contains a detailed examination of each of the four attributes that Aristotle identifies as belonging to different orders of besouled creatures. The first, with belongs to all life forms, is nutrition and reproduction. That is, it has a way of taking in food and using that food to continue its existence through offspring. The second, which belongs only to animals, is sensation. The third, which belongs only to higher animals, is local movement. Aristotle also hypothesizes that appetite is a necessary condition for local movement and imagination for appetite, so all three attributes will necessarily be found together. Finally, humans alone, of all besouled creatures known to Aristotle, have the attribute of reason.
As is typically the case with Aristotle’s scientific works, De Anima is well organized and the conclusions are logical based on the facts he had to work with. Numerous details are completely wrong because of his reliance on the “four elements” model of physical science and lack of all but the most basic scientific instruments. Nonetheless, at least from a biological point of view, his general definition of the soul and the attributes of organisms with souls still seem perfectly plausible.