e may deny
that a leaf is green on one side without being bound to affirm that it
is not-green on the other. But in the same relation a leaf is either
green or not-green; at the same time, a stick is either bent or
not-bent. If we deny that A is greater than B, we must affirm that it is
not-greater than B.

Whilst, then, the principle of Contradiction (that 'of contradictory
predicates, one being affirmed, the other is denied ') might seem to
leave open a third or middle course, the denying of both
contradictories, the principle of Excluded Middle derives its name from
the excluding of this middle course, by declaring that the one or the
other must be affirmed. Hence the principle of Excluded Middle does not
hold good of mere contrary terms. If we deny that a leaf is green, we
are not bound to affirm it to be yellow; for it may be red; and then we
may deny both contraries, yellow and green. In fact, two contraries do
not between them cover the whole predicable area, but contradictories
do: the form of their expression is such that (within the _suppositio_)
each includes all that the other excludes; so that the subject (if
brought within the _suppositio_) must fall under the one or the other.
It may seem absurd to say that Mont Blanc is either wise or not-wise;
but how comes any mind so ill-organised as to introduce Mont Blanc into
this strange company? Being there, however, the principle is inexorable:
Mont Blanc is not-wise.

In fact, the principles of Contradiction and Excluded Middle are
inseparable; they are implicit in all distinct experience, and may be
regarded as indicating the two aspects of Negation. The principle of
Contradiction says: _B is not both A and not-A_, as if _not-A_ might be
nothing at all; this is abstract negation. But the principle of Excluded
Middle says: _Granting that B is not A, it is still something_--namely,
_not-A_; thus bringing us back to the concrete experience of a continuum
in which the absence of one thing implies the presence of something
else. Symbolically: to deny that B is A is to affirm that B is not A,
and this only differs by a hyphen from B is not-A.

These principles, which were necessarily to some extent anticipated in
chap. iv. Sec. 7, the next chapter will further illustrate.

Sec. 6. But first we must draw attention to a maxim (also already
mentioned), which is strictly applicable to Immediate Inferences, though
(as we shall see) in other kinds of proof it may be o