Without doubt it is not possible to understand Descartes' metaphysics without some knowledge of his physics. We must remember that the aim of this metaphysics is to offer a secure foundation to Descartes' own mechanistic science of nature, which he believed was a far superior science than the Aristotelian view of the universe that he had been taught as a student. In a very simple way, Aristotelian metaphysics views each thing as an individual substance, and to understand a thing is therefore to be able grasp its individual essence. To understand a horse is therefore to be able to understand what it means for x to be a horse, to understand the individual substance ‘horse’. The 17th century mechanistic view of nature is a complete rejection of individual substances as the basis of scientific explanation. Of course this does mean that they did not think that individual things, like horses, did not exist, but that we needed, in order for it to be truly scientific, an explanation at a ‘deeper’ level. Here the inspiration is not Aristotle, but the classical Greek atomist like Democritus, Leucippius and Epicurus. We understand a thing not by describing what that thing is, but by analysing the number, shape and motion of the atoms that make it up. Atoms are common to all things, and not just to one individual substance. The world is therefore ‘nothing but matter in motion’. Thus Descartes can write in the Principles of Philosophy that all the phenomenon of nature can be explained by the ‘the shape size, position and motion of particles of matter.’
This is one of the most significant and important differences between the substance of matter and the substance of mind for Descartes. Let us remind ourselves of Descartes’ metaphysics of substance. Descartes takes the Aristotelian definition of substance to be that which is independent. In other words that which is not dependent on anything else in order for it to exist. From this definition the only independent substance is God, since everything else, as we have seen in the third meditation, depends on God in order to exist. We then must distinguish between infinite substance and finite substance. We might see this as a theological and metaphysical description. Descartes combines this distinction, however, we find another Aristotelian definition of substance, as that to which we predicate properties, attributes or qualities. Substance, in this sense, is the subject of a judgement or proposition. We can distinguish between what Descartes calls ‘real distinctions’ and 'modal distinction'. A real distinction is a distinction between substances. Now for Descartes, there are only two kinds of substance, mental and material (God is not another kind of substance, for God is a infinite mental substance). A modal distinction, on other hand, is a distinction between a substance and one its attributes. To distinguish between Socrates and his white skin, is a modal distinction. To distinguish between matter and mind is a real distinction .
In a certain sense, Descartes has not really left Aristotelian metaphysics with these distinctions. The great difference between Aristotle and Descartes is that he does not believe, as we have said already, that substantial forms sufficiently explain what something is. In other words, for Descartes, to know what a horse is, knowing the definition of a horse, is not a sufficient explanation. To really know what a horse is I would have to understand the mechanical operation of its body parts. This means that unlike Aristotle, the difference between individual substances is modal and not real. It is the same explanation for a horse as it for a man. This, however, is true only of material substance, and not mental substance. For mental substance, the contrary is the case. Mental substance is individual. Thus my thinking, even if we are thinking the same thought, is not the same as your thinking. As thinking things, we are all unique individual substances. Descartes thinks that this must be the case because I can think of myself entirely outside of thinking any other mental or material thing. Thought by definition, for Descartes, is individual. This would mean that unlike material things, the difference between one thinking individual and another is real rather than modal. I am unique as you are unique, unlike a piece of lead which is just lead, and ultimately a certain arrangement of material atoms. This difference between the individuality of mental substance, and the generality of material substance is the basis of Descartes metaphysical dualism, and is one of the ideas Spinoza will reject. Descartes writes about the individuality of mental substance:
From the mere fact the each of us understands himself to a thinking thing capable, in thought, of excluding from himself every other substance, whether thinking or extended…. It is certain that each of us, regarded in this way, is really distinct from every other thinking substance and from every corporeal substance.With this background in mind, let us begin to read the fifth meditation. What we need to understand is that the notion of an object as made of particles of matter is an idea, and not something that we obtain from our senses. The problem for Descartes now is, and will be for the next two meditations, how we can return to the certainty of material objects that we denied in the method of doubt:
The most pressing task seems to be to try to escape from the doubts into which I fell a few days ago, and see whether any certainty can be achieved regarding material objects [AT VII 63].Now Descartes' method here, as before, is not jump simply to the object outside of himself and ask whether they exist in themselves can be known by the senses, but to consider the idea of a material object that he has already in his consciousness:
I must consider the idea of these things, in so far as they exist in my thought…’ [AT VII 63].Now in a certain sense, we have already come to this argument before in the second mediation, when Descartes considered the piece of wax. We remember that his argument there was to shown that our true understanding of the wax cannot come from our senses, but from the idea of wax itself as something that is capable of extension. I think what Descartes is saying is that to experience an object at all we need an idea of an object and this idea must precede our experience, for it explains what our experience is of. Without this idea of an object our senses would be extremely confused and hazy. All we would have is a list of different events, but we would have no coherence or unity of experience. It is only in the fifth meditation that Descartes brings out fully what the idea of such object might that would support my experience:
Quantity, for example, or ‘continuous’ quantity as the philosophers commonly call it, us something I distinctly imagine. That is, I distinctly imagine the extension of the quantity (or rather of the thing which is quantified) in length breadth and depth. I also enumerate various parts of the thing, and to those parts I assign various sizes, shapes, positions and local motions; and to the motions I assign various durations [AT VII 63].The idea of a material object, therefore for Descartes is mathematical, or more precisely geometrical, and this fits in exactly with his scientific theory. It is also for this reason that he introduces the notion of ‘immutable essences’. Of course for Descartes, these essences belong, as they do for Plato, not to the external world, but to the idea of the object itself. Not the idea of an object as the representation of something, but the immutable essence that belongs to the idea itself. What Descartes has in mind here is that there some ideas in my mind which of necessity belong to my conception of object, whether in fact this object exists or not. The example the Descartes uses is that of the idea of a triangle:
When, for example, I imagine a triangle, even if perhaps no such figure exists, or has existed, anywhere outside of my thought, there is still a determinate nature, or essence, or form of the triangle which is immutable and eternal, and not invented by me or dependent on my mind. This is clear from the fact that its three angles equal two right angles; that its greatest side subtends its greatest angle, and the like [AT VII 64]It is very important that we do not confuse this necessity with actual triangles, rather it belongs to idea of the triangle itself. The idea of the triangle tells nothing about whether external triangles exist or not outside of my mind. This is significant, because it tells us that for Descartes there is no direct route from the mind, and from the ideas that are contained in the mind, to the existence of external thing. There is, however, one idea that is different from all the other ideas and that is the idea of God. This should not surprise us from the Third Meditation. In that mediation, we argued from the causality of the ideas; that is to say, that I could not be the cause of the idea of God, since I am an imperfect being, and imperfection cannot be the cause of perfection. Here, Descartes argues not from the origin of the idea, but from its definition. Just as the idea of the triangle necessarily contains the idea of 3 angles, so the definition of God necessarily contains existence.
How could this be so? Normally, Descartes says we would distinguish essence from existence. As we have said, just because the essence of the triangle contains the idea of three angles this does not prove that the anything like triangles exist in the exterior world:
Since I have been accustomed to distinguish between existence and essence in everything else, I find it easy to persuade myself that existence can also be separated from the essence of God, and hence that God can be thought of as not existing [AT VII 66].But in the case of God existence is a necessary predicate of the idea, just as in the case of triangle three angles is a necessary predicate. This does not mean for Descartes that triangles also necessary exist, for there is nothing about the idea of three angles to the idea of triangle that would mean that such things would have to exist outside of my idea of them. With the idea of God, however, the existence of the object that corresponds to the idea is necessary. Why is this the case? This is because the idea of God contains perfection, and I would be contradicting myself to say that God is perfect and does not exist, since non-existence is perfection.
Such argument, as we know is not novel. What is important, however is how the necessary existence of God fits within Descartes' overall metaphysical system. It is the existence of God that guarantees the existence of external objects, and also that my idea of these objects correspond to external objects. What I can clearly and distinctly perceive is true, but without God this truth would not be sufficient, since I can always that although I am perceiving this truth in my mind, there is nothing that corresponds to it in the outside world. If I can prove that God exists, then it follows that everything depends upon him, since God is the only perfection, and such a God could not deceive me. Then what I clearly and distinctly perceive, and I can remember having done so, must be actually true:
I have perceived that God exists, and at the same time I have understood that everything else depends on him, and that he is no deceiver; and I have drawn the conclusion that everything which I clearly and distinctly perceive is of necessity true [AT VII 70].