Aristotle does not begin his treatise On the Soul by declaring that we are made in the image of God.
Rather, he begins with the stunningly obvious observation that among all of the things which exist, we distinguish those which live from those which do not. Each of those things which live (plants, animals, and human beings) have living bodies which are constituted by various organs and heterogeneous parts which yet function as a whole, serving that one living being.
What can be the causal principle of these organic unities? What is the principle of life?
This principle, whatever it turns out to be, is what we call a soul!
It is from this humble beginning that he kicks off the true beginning of his treatise. It is from a simple, empirical fact (that there are living beings and that something makes them live and function as complex unified organisms) that we can state there must be some principle or cause of this fact–a principle of life–which is nothing but what we mean by soul.
Much follows from this little observation, this humble step in reasoning which simply acknowledges what is obvious to every human being (that animals, plants, and people are uniquely and specially distinct from rocks or water, etc.). But having laid this firm ground work, Aristotle prepares his readers to discover much about the wondes of ‘ordinary’ reality, the mystery of life.