On the Soul
After presenting the views of previous thinkers, Aristotle proceeded to develop his own theory in the rest of the work. Aristotle's arguments rest on a handful of concepts that are central, not only to the Physics, but to the rest of his philosophy. One is his distinction between Matter and Form. Applied to this subject, Aristotle would call the body matter and the soul the form; they therefore are indivisibly linked as potentiality and actuality. A second concerns his theory of sense perception in which the process is described as receiving in the sense organs of the perceiver "sensible forms" thrown off by the objects of perception. A third is his distinction between the active and the passive intellects. As Aristotle focuses more on the first of these points, so shall I. After arguing that the soul is a substance in the sense of Form, he gives his definition of soul; "the first actuality (entelechy) of a natural body with organs." While this implies that there is a profound unity between soul and body, Aristotle does not provide us with any idea of if they are at all distinct or separate. What he does do is distinguish the various faculties of the soul; the nutritive, perceptive, and intellective faculties. These together with movement create his definition of an animal. There is, however a hierarchy among the faculties of the soul and the soul of the plant differs from that of an animal or man only to the extent that it possess or does not posses these qualities.
Now that a summary of Aristotle's opinions on the soul has been presented, it is necessary and proper for us to consider how his concepts coincide or differ to the traditional Catholic idea of the soul. As far as the Soul is the Form of the body, Aristotle is in complete union with Catholic teaching. The Catechism of the Catholic Church declares in paragraph 365 "The unity of soul and body is so profound that one has to consider the soul to be the 'form' of the body: i.e., it is because of its spiritual soul that the body made of matter becomes a living, human body, spirit and matter, in man, are not two natures united, but rather their union forms a single nature." A single nature, that is the key phrase. There is no duality here (which is a heresy, btw), which was advocated so strongly by Plato, but rather Aristotle calls the soul the actuality of the body, which is to say that the body has forms which are potential in it, i.e., dead or alive, and that the soul is the actualized form of alive of the body.
But this sounds suspiciously like Aristotle is just speaking about life in general. We are free to infer from him what we will about immortality, and it would even be plausible to say that the Soul ceases to exist after the body ceases to exist, as it is absurd to speak of potential energy in a non-existing system, so it is to speak of a soul for a non-existing body. This will require more thought and much deliberation, but at a later date.