5. Of abstract relations between ideas.
Secondly, the next sort of agreement or disagreement the mind perceives in
any of its ideas may, I think, be called relative, and is nothing but the perception of the relation between any two
ideas, of what kind soever, whether substances, modes, or any other. For, since all distinct ideas must eternally be
known not to be the same, and so be universally and constantly denied one of another, there could be no room for
any positive knowledge at all, if we could not perceive any relation between our ideas, and find out the agreement
or disagreement they have one with another, in several ways the mind takes of comparing them.
