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

40
THE NAME, THE TERM, THE CONCEPT,
tradiction, 1 was thus stated by Aristotle :
"
the proposi
tions A is B and A is not B cannot both be true
together" The third law, now known as the Law of
Excluded Middle, was formulated by Aristotle thus :
"
of
the two propositions A is B and A is not B, one must
be true and the other false."
ii. As it stands, the Law of Identity, "A is
A,"
does not give us any information. It may, however, be
interpreted so as to make it a genuine principle on
which the very life of Thought depends.
(a) We have seen that in actual thinking we require
terms to identify our thoughts. The Term identifies a
"
"
universal meaning
( 6). The Law of Identity has
an important application to this relation. Let A denote
anything thought about, any more or less defined idea
which is distinguished from other ideas so far as to be
indicated by a single symbol in language, a name or
term, M. Then to say that "A is A" means that M
must always stand for the same A, the same for
different minds and for one mind at different times.
Terms must have fixed meanings, each clear in itself
and distinct from others. If the meaning of a term
is changed, it should be done deliberately and for a
sufficient reason.
(V) In another sense, the principle means that what is
true must be consistent with itself; and this is one of
the necessary tests of truth. This principle was laid
down by Aristotle, though he does not attempt to cast it
into the form of a Law of Identity (An. Prior., i. 32) :
"All truth must be consistent with itself in every
direction." Aristotle is here thinking specially of the
consistency of a conclusion or consequence with the
1
Sometimes referred to, more
appropriately, as the Law of Non^
contradiction,