In order to study more easily the nature of the idea in Descartes, we can distinguish its relations to the mind and the object.
Its relation to the mind provides what Descartes calls the formal reality of the idea (and, at least in one text, it is the same thing that he intends by the material substrate of the idea).
Descartes understands its relation to the object in two very different ways: sometimes the object itself, as it is in the mind, is conceived as being the objective reality of the idea. Sometimes this objective reality is conceived as being the representation, the image of the object. The first theory gives rise to the thought of Arnauld, as well as to Berkeley. The second is that of Malebranche.
On one hand the idea of the object is the object itself (this is the view of James, Bergson, the empirio-criticists, and the neorealists). On the other hand the idea is separated from the object (this is the idea of intentionality in phenomenology and the American critical realists).
It is not easy to sacrifice one of these conceptions for the other: such is the antinomy of realism.
Antinomy, of course, does not mean falsity.
It is even one of the grandeurs of Descartes, the “father of modern idealism,” to have expressed the two fundamental and antinomical conceptions of realism.
With Maritain I do not consider it just to accuse Descartes of having invented a doctrine of idea-images.
The idea of the sun is not the visible sun; it is the sun of the astronomers, which is drawn from the consideration of innate ideas. The clear and distinct idea is the innate idea.
On the separation and union of the soul and body in Descartes: it has not been sufficiently said that both are demonstrated in the same way. By the faculties: intellection proves the distinction; sensation and imagination prove the union. By nature: the soul is indivisible; matter is divisible (this proves the distinction); yet the soul is indivisible and the body is indivisible (this allows the union). And both are operated by the power of God: God can separate what is distinct, and the same God can unite what is distinct. The God who separates what is distinct is God as the foundation for and the means toward the in-depth exploration of clarity and distinction. The God who unites what is separated is the God who wants to realize a complete man, a being in itself, through uniting the substances, complete in themselves, that are extension and thought.
And just as experience gives the essences as well as this intellectual experience that is the vision of clear and distinct ideas, it cannot be said that the union is demonstrated any less firmly than the distinction. The proof is that experience truly gives a substance—a third substance which comes to be added onto the two clear and distinct ideas of the soul and extension.
As he unites, God creates a separated substance.
Descartes is so strongly in favor of separations that, even when he represents a union, he insists on the fact that this union is not a distinct idea without doubt, but an idea distinguished from what he composes.
Of the three kinds of ideas distinguished by Descartes, the ones that are innate in us allow us to escape from ourselves.
Here the idea and judgment are hardly separated any longer. An innate idea is a judgment. “It is virtually the same thing to conceive the idea of God and to conceive that he exists.”
Descartes’s tendency is doubtlessly to reduce the efficient cause to the formal cause, causa sive ratio, to see the idea of cause as being equally applicable to the nontemporal as the temporal (idea of the causa sui). This is why he will be able to apply the principle of causality to the ideas. In appearance, Descartes seeks for the cause of the image; in reality, he seeks the reason of the idea.
But he well knows that the efficient cause is not exactly the formal cause. It can simply be treated as the formal cause, in the same way as the formal cause can be treated as an efficient cause (causa sui).
Concerning the meeting of the tendencies to fuse notions and to separate notions: these give rise to certain ideas proper to Descartes: in the Regulae, the idea that the simple natures can confine relations, and later the idea of the third substance and the idea of continuous creation.
Descartes wants to melt the past, present, and future into this instant where the mind will possess the truth without reasoning, but he also wants to mark a certain distance between things, to stretch them out; this distension is the space that makes the idea in us only like an image of things, and a trace of God, and makes the efficient cause to be never like a quasi-formal cause.