nother. Nor can either
of these functions of thought be separated from the other: as
Aristotle himself said, the knowledge of opposites is one. A
thing which has nothing to distinguish it is unthinkable, but
equally unthinkable is a thing which is so separated from all
other things as to have no community with them. If then the
law of contradiction be taken as asserting the self-identity
of things or thoughts in a sense that excludes their
community--in other words, if it be not taken as limited by
another law which asserts the _relativity_ of the things or
thoughts distinguished--it involves a false abstraction....
If, then, the world, as an intelligible world, is a world of
distinction, differentiation, individuality, it is equally
true that in it as an intelligible world there are no absolute
separations or oppositions, no antagonisms which cannot be
reconciled."[2]

In the penultimate sentence of this quotation Dr. Caird
_differentiates_ his theory against a Logical counter-theory of
the Law of Identity, and in the last sentence against an Ethical
counter-theory: but the point here is that he insists on the relation
of likeness among opposites. Every impression felt is felt as a change
or transition from something else: but it is a variation of the same
impression--the something else, the contrapositive, is not entirely
different. Change itself is felt as the opposite of sameness,
difference of likeness, and likeness of difference. We do not
differentiate our impression against the whole world, as it were,
but against something nearly akin to it--upon some common ground. The
positive and the contrapositive are of the same kind.

Let us surprise ourselves in the act of thinking and we shall find
that our thoughts obey this law. We take note, say, of the colour
of the book before us: we differentiate it against some other colour
actually before us in our field of vision or imagined in our minds.
Let us think of the blackboard as black: the blackness is defined
against the whiteness of the figures chalked or chalkable upon it, or
against the colour of the adjacent wall. Let us think of a man as a
soldier; the opposite in our minds is not the colour of his hair,
or his height, or his birthplace, or his nationality, but some other
profession--soldier, sailor, tinker, tailor. It is always by means of
some contrapositive that we make the object of our thoughts