From: John Baima (email@example.com)
Date: Thu Jun 28 2001 - 17:04:50 EDT
At 08:17 PM 6/28/2001 +0000, firstname.lastname@example.org wrote:
>I have seen it stated that it is a logical fallacy when one assumes that a
>conditional statement is also true in the reverse as in "If A, then B is
>not equal to if non-A, then non-B) as described by Wallace on page
>687. Could one also state that this is also the case for "if not B then
This is right. If you have a sentence of the form, "if A then B" there are
two ways to make a valid deduction from the sentence (valid does not imply
a sound deduction).
1. If "A" is true then you know "B" is true (modus ponens "method of
affirming" inference rule)
2. If "B" is false then you know "A" is false (modus tollens "method
of denial" inference rule)
It is also therefore true that the two sentences are logically identical:
1. If A then B
2. If not B then not A
Rational authors never assume the protasis ("if portion") false for the
sake of argument because no infrence can be made from a false protasis.
Sometimes they are trying to prove that the protasis is false. For example,
I found an example in the papyri that went, "if my client was guilty then .
. ." as part of a courtroom speech. The lawyer was trying to prove that his
client was not guilty, not assume that he was guilty for the sake of argument!!
>Assume a third class conditional with EAN + subjunctive followed by an
>apodosis in the future.
I'll let someone else talk about that.
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