mikew@watarts.UUCP (03/07/84)
Ms. Creighton confuses two principles of logic in
her discussion about temporal statements and their
truth values. She argues that certain statements, that
are meaningful now, would have been meaningless at some
prior time. She then concludes that these statements
are neither true nor false. Whether this is so is not my
concern. However, if these statements were meaningless then
they cannot be bearers of truth values. In order to be
a candidate for a truth value, you must be at least meaningful.
Thus, if Ms. Creighton argued that 'grxxxrt' did not
have a truth value because it was not well-formed no one
would care. Neither should anyone care that statements
that are unintelligible might not have truth-values.
So far Ms. Creighton has argued, unsuccessfully in my
view, that we should reject the Law of Bivalence for temporally
tagged sentences. The Law of Bivalence is the claim that
there are only two truth values, true and false. Ms. Creighton
would seem to want to give certain temportal statements some
different value. I think her argument unconvincing.
However even if it was convincing, we can reject the
Law of Bivalence and still keep the Law of the excluded middle.
The latter simply allows one to infer P from --P. In fact,
the two laws are independent of each other. In some logics,
intuitionistic logic for example, bivalence is true but
the inference --P implies P fails. In some many-valued
logics bivalence fails, but the inference is valid.
Michael Webster Dept. of Philosophy U.W.