Discourse on Method and Meditations on First Philosophy

Descartes relies on a key scholastic distinction between Formal reality and Objective reality. He writes:

I must not doubt either that it is necessary that reality be formally in the causes of my ideas, although the reality which I consider in these ideas be only objective, nor think it sufficient that this reality be found objectively in their causes (Meditation 3, 120).

By objective reality, Descartes understands the ideas we have regarding certain phenomena in the world. Moreover, Descartes subverts the original meaning of this term given to it by Plato, who used Form and Idea interchangeably to speak about the true essence or nature of things.

Thus, the implication of this line of thinking is that things that are considered objectively real are said to be so because they are products of cognition. By contrast, formal reality denotes the mind independent existence of the world. That is to say, to know the formal reality of things is to know a given phenomenon in such a way that we understand what it is, regardless of what one may think or feel about it, hence the mind-independent character of formal reality, in contrast to the mind-dependent nature of objective reality, insofar as they are products of human cognition and can only exist insofar as human cognition exists.