Aristotle (384-322 B.C.)

Unlike his teacher Plato, Aristotle was a naturalist believing in only one world, the natural world, and not any separate supernatural realm.   However, Aristotle's idea of nature was more 'supernatural' than modern ideas tend to be.

Also unlike Plato, Aristotle was an empiricist, believing that philosophy should start with what we sense or experience (although of course it is also important to use reason and logic).

Perhaps the biggest difference between Aristotle's conception of nature and the contemporary scientific one is that Aristotle's view was teleogical, i.e. it included the idea of a purpose or goal (telos in Greek) for all kinds of natural thing.  Each thing has an entelechy or inner drive to achieve this purpose.

Like Plato, Aristotle believed in forms, but, being a naturalist, he thought of these as part of the natural world, existing within objects, not separate from them.  Every body has a form, and every form needs a body to exist in.  For Aristotle, a thing's form is its essential or defining characteristics.  For a shape this would be its form in the literal sense.  For other things it might include shape, certain activities, context, etc.  Whatever it is about a thing in virtue of which it is that kind of thing rather than some other is its form.

For a living thing, the form is what is called the soul.  So all living things have souls, according to Aristotle.  Nutritive or vegetative souls consume and grow.  Sentient souls (as in animals) sense or feel.  Rational souls (as in human beings) think.  These are the characteristic activities that define living things of each sort.

To understand anything completely one must know its four "causes."  The material cause is the matter of which it is made.  The formal cause is its form.  The efficient cause is what produced it (which we would usually call the cause).  The final cause is its purpose.

God (another surprising part of Aristotle's non-supernatural world) is the ultimate final cause.  Starting with ordinary observation, Aristotle notes that things move, and that the source of their movement is some other moving thing.   If what efficiently causes x to move is always some other thing y, then there can never be an original efficient cause of movement itself.  Movement in the world must be eternal.  But there must be some reason why there is movement at all.  If not an efficient cause, this reason must be a final cause.  And since the movement in question is eternal, its final cause must be eternal.  So there must be an eternal cause of all movement.  Aristotle calls this prime mover God.

But what is God?  Obviously God is not material (that would be absurd and degrading), so he must be spiritual or intellectual (a bit like Plato's forms in fact).  As a superior, intellectual being, he must not lower himself to physical acts but instead must only ever use his intellect, i.e. think.  And, being perfect, he must think only of what is best, i.e. himself.  So Aristotle's God is thought thinking thought.