Thinking is something that is intrinsic to each individual. It is an activity one person cannot share with another. That one has eyes that see and legs that walk is obvious, but what part of the person is it that thinks? That is not obvious. For Aristotle it was the mind that did the thinking. To think of something, he said, was for the mind to become what is thought of. Thus he said that mind was made up of thoughts. Prior to thinking the mind was nothing. In fact it did not become something of its own until it thought of itself. However, to think of something is not the same as to have knowledge concerning it.

One can think of what one has encountered in experience. But experience, in an Aristotelian sense, is only of things and things are constantly changing, knowledge can only of what can not be other than it is. In other words of what is not changing. As a result knowledge must be extracted from experience, from our experience of things. Aristotle's great works on logic were designed to explain how we can accomplish this extraction.

Plato's concept of knowledge is not of experienced things at all, but of the unchanging ideal forms. They cannot be known directly in this world, because everything in this world is constantly changing. Experiencing something merely reminds us of the ideal forms. These are developed through recollection by the soul of the forms that were known prior to its entry into the body. This makes that part of the person that does the thinking divine as well as a part of its soul.

Augustine's theory of knowledge merely translated the Platonic forms into exemplars in the mind of God that could be known through divine illumination. The merging of these two lines of thought in the thirteenth century was accomplished by Aquinas. He did this through what he called the "active intellect." This, he said, was the highest faculty of the soul. The ray of Augustinian divine light that enables the soul of man to grasp the eternal forms which constitute the intelligible structure of the universe.

A pure intellect would be able to experience eternal truth directly, but humans are composites consisting of soul and body, form and actuality. As such man gets his first contact with the external world through the senses and this gives him the incentive to begin thinking. For Aquinas that by which the individual thinks is the intellectual soul, a concept derived from Aristotle's concept of mind. It is only following experience and through the intellectual soul that the active intellect can extract the ideal forms from sensible experience.

. The process of understanding is the reduction of experienced things to intelligible terms. It is a progressive discovery of the eternal forms in the changing things of sensual experience.

Finally, the whole intelligible structure and ultimate explanation of the universe, abstracted from the sensible medium in which it is first conveyed to us, fills the intellect and fulfills the intellect's capacity for knowledge, enlightening it with the vision of absolute truth, in so far as a finite and embodied mind can be enlightened.

If we follow the development of Thomistic Scholasticism to the end of the sixteenth century we find that its major exponent was the Jesuit philosopher theologian Francis Suarez. A good example of the kind of thinking this approach leads to can be shown with this description of Suarez' answer to the problem of contingency and possible essences, the underlying feature of Leibnizian metaphysics, as expressed by a modern Jesuit,. Frederick Coppleston.

Suarez' view is, then, this. 'Because existence is nothing else than essence constituted in act, it follows that, just as actual essence is formally limited by itself, or by its own intrinsic principles, so also created existence has its limitation from the essence, not because essence is a potentiality in which existence is received, but because existence is in reality nothing else but the essence itself.

In other words, since the essence of something that exists includes the mode of the things existence, there can be no essence of something that does not exist. This eliminates the possibility of possible existences. But in a world where every possibility is known by God, and where the essence of something includes its total existence, the denial of possible essences denies the possibility of contingency. In other words nothing can be different than it is. The problem of possible essences was avoided by Descartes as he avoided many other Scholastic arguments by his different method of pursuing understanding. Spinoza tried to show that possible essences were an absurdity because everything that exists does so through the nature of God. But arguing in this fashion also denies the existence of contingency. Descartes never gave contingency any thought and Spinoza rejected the very idea of it. However, for man and God to be free in the sense that they can be judged by the actions they choose to make, contingency is essential. As we saw in the last chapter Leibniz returned to the existence of possible essences with his plenum of complete individual conceptions.

However, across the English channel they developed an entirely different attitude. There the question became "So what?" Questions of this kind cannot be solved by pure reasoning thus they are not important. What is important is knowledge that can change the way men live their lives, like the invention of Gunpowder, the printing press, the magnet, and other practical sciences.