An introductory text-book of logic - Mellone, Sydney - Rare Books at ...

AND THE LAWS OF THOUGHT. 43
the same predicate should both belong and not belong
to the same thing at the same time and in the same way"
A thing may have different qualities at different times,
as in the changes in a person s character ; and it may
have a quality in one respect, and not have it in
another, as in the celebrated shield that was gold on
one side and silver on the other ; but these facts do not
conflict with the law of Contradiction as Aristotle states
it. Aristotle points out that the denial of this principle
would be the denial of the very possibility of thinking.
13. The law of Excluded Middle says that of the
"
"
"
two propositions A is B and A is not one must
B,"
be true and the other false. In this form the principle
and the student will observe
was laid down by Aristotle ;
its close connection with the principles of Identity and
Contradiction as regards the meanings of terms and the
consistency of propositions. The application of the
principle is plain in proportion as A and B are exactly
defined. If we are in doubt as to where one thing
begins and another ends, we are in doubt as to the
precise application of our principle. This may happen
in cases where we do not find a perfectly definite limit
to an event in space or time e.g., when something is
"in the act" of occurring, we seem unable to say,
"
either it has happened or it has not The
happened."
sun may be just "rising" without "having
"
not having
a precise meaning to
risen" or
risen." But as soon as we have attached
"
rising," in the case of the sun,
e.g., if we make it mean that the actual globe
is visible
above the true horizon, then the law of excluded
middle is applicable. When we are speaking of natural
qualities such as heat, which always have degrees, then
again we cannot say that a body must either
"
be
"
hot
or
"
not be
"
hot until we know that some definite