According to Aristotle, Averroes and Aquinas there are for things two ways of existing: “One and the same thing exists in a certain way in nature and in another way in the soul. And its existence in the soul is one with the act of cognition. . . . Whereas for Descartes, in the soul you do not have a thing, you have an idea of it. What is decisive here is this, that this idea of the thing is known first, as if, not having had the pleasure of meeting you, I have seen your photograph and from the features of the photograph I infer what you are in reality. The relation is causal here. Not one of intentionality but one of causality. . . . That which is understood, in Descartes, is a psychological disposition, an idea. What causes us to rely on that psychological disposition, that idea? Provided you master Descartes’ four rules of the method, you can differentiate between ideas that are dependable and those that are not. The ideas that are dependable are so because they are really occasioned by things. The ideas that are not dependable are not occasioned by things. This Cartesian principle, that what we know primarily are not things but rather our ideas of them, is a fundamental principle of idealism.”
~Yves R. Simon: “The Great Dialogue of Nature and Space,” Chap. I.
In "The Great Dialogue of Nature and Space," Yves Simon "discusses the basic insights of the creators of modern thought: Descartes, Newton, Galileo, Comte, Mach, Meyerson, Bergson, Planck, and the issues at stake in the development of modern science and in the rejection of the Aristotelian physics."http://en.wikipedia.org/wiki/Yves_Simonhttps://www.facebook.com/StThomasofAquin?fref=nf