Does "If A then B mean if not B then not A" ?

DJNordlund at aol.com DJNordlund at aol.com
Thu Jun 28 21:17:49 EDT 2001


In a message dated 6/28/2001 1:14:33 PM Pacific Daylight Time, 
rickstamp at gmx.net writes:


>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 not A"?    Assume a third 
class >conditional with EAN + subjunctive followed by an apodosis in the 
future.

>Sincerely,
>Rick Stamp

Rick,

when you have a valid implication such as

      (1) if A then B

you are correct in understanding that it is NOT logically valid to claim

      (2) if not-A then not-B

However, if (1) above is valid, then it IS valid to conclude

       (3) if not-B then not-A

regardless of the Greek you are analyzing.

Hope this is helpful,

Dan Nordlund


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