Many of the things noticed in this chapter, especially in this section
and the preceding, will be discussed at greater length in the chapters
on Classification and Definition.

§ 7. Contradictory Relative Terms.--Every term has, or may have, another
corresponding with it in such a way that, whatever differential
qualities (§ 5) it connotes, this other connotes merely their absence;
so that one or the other is always formally predicable of any Subject,
but both these terms are never predicable of the same Subject in the
same relation: such pairs of terms are called Contradictories. Whatever
Subject we take, it is either visible or invisible, but not both; either
human or non-human, but not both.

This at least is true formally, though in practice we should think
ourselves trifled with if any one told us that 'A mountain is either
human or non-human, but not both.' It is symbolic terms, such as X and
x, that are properly said to be contradictories in relation to any
subject whatever, S or M. For, as we have seen, the ordinary use of
terms is limited by some _suppositio_, and this is true of
Contradictories. 'Human' and 'non-human' may refer to zoological
classification, or to the scope of physical, mental, or moral powers--as
if we ask whether to flourish a dumbbell of a ton weight, or to know the
future by intuition, or impeccability, be human or non-human. Similarly,
'visible' and 'invisible' refer either to the power of emitting or
reflecting light, so that the words have no hold upon a sound or a
scent, or else to power of vision and such qualifications as 'with the
naked eye' or 'with a microscope.'

Again, the above definition of Contradictories tells us that they cannot