it is one of the only principles about which one can be certain, and at the same time one can not see at all what it offers to us. It is certain, but it is empty. This famous principle is the principle of identity. The principle of identity has a classical formula, A is A. That is certain. If I say blue is blue or God [is] God, I am not saying with this that God exists, in a sense I am in certainty....Let us nonetheless try to say what results from this principle of identity. It is presented in the form of a reciprocal proposition. A is A means: subject A, verb to be, A attribute or predicate. There is a reciprocity of subject and predicate.