Leibniz intellectualized appearances while Locke sensualized the concepts of the understanding. The principle of the identity of indiscernables is based on a presumption that if a certain distinction is not found in the concept of a thing in general, it is not found in the things themselves. According to mere concepts, the inner is the substratum of all relational or outer determinations. Thought is in itself no product of the senses and is not limited by them. But it does not follow that it has a pure employment of its own, for then it is without an object.
Examples are pure space and pure time.