“[E]very understanding which I posit as different from my own, is only a position of my own understanding, i.e. an idea of my own, a conception which falls within my power of thought, and thus expressed my understanding. … What I think of as united, I unite; what I think of as distinct, I distinguish; … [M]y understanding or my imagination is itself the power of uniting … ; understanding is only the understanding which exists in man[.]”