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Re: ex falso quodlibet



Sorry, not that I doubt about your distinction, but I can't remember now
any counterexample of the following:
isn't it true in propositional logic that any false statment, being a
proper statement, is actually a contrdiction?
I guess this is a matter of definition. While a contradiction is a
sentence which is false for any interpretation, to say that a sentence is
false doesn't necessarily implies this, for this can depend on the
interpretation.

Best regards,
Miguel A. Santos
msantos@etse.urv.es

On Thu, 28 Oct 1999, Richard Grandy wrote:

There is an important distinction to be marked between "ex falso quod
quodlibet" and "ex contradiction quodlibet". The first says that from a
false statement anything/everything follows, which is not a principle of
symbolic logic. The second says that from a *contradiction*
anything/everything follows, which is an accepted principle.


Richard Grandy
Philosophy
Rice University

John: Thanks for specifics of the reference. I will look there.

John pointed out (in reference to applying logic to judging people):
That seems like quite a leap.
If the other person is offering proofs and syllogisms based on a
self-contradictory premise, then you *should* reject everything that
follows. _Ex falso quodlibet_ is not an assumption -- it's a theorem of logic.

In my statement about my physics colleagues, I was careless in my wording.
What I meant is that many in our profession utilize their deductive
reasoning skills to dismiss anything which does not fit the data already
assimilated by them.

In the case of lectures (say at AAPT meetings) it is all too common for
members of the audience to attempt to discredit the entire talk by "looking
for" the premise to discredit. Specifically, in the case of "They're not
dumb, they're different" one can escape culpability for ingoring all but
the ones "like us" if one can discredit the initial premise. So what I
observed was an audience whose ears and minds were closed because they were
too busy working backwards from their own forgone conclusion in an attempt
to discredit the initial premise.

I am equally guilty of the deductive reasoning error. When I conclude 2 +
2 = 4 and discover the solution yields incorrect predictions, I simply
re-add, with the result that 2 + 2 = 4, again. Frequently, however, I have
overlooked additional components, such as 2 + 2 + 5 + 7 so that my logic
still produces 4 for the answer. This not only happens to us socially, but
in our research, as well. Sometimes we get lucky and have figured out
*all* of the variables and controlled them. Sometimes the other variables
are insignificant. Other times, we jump to conclusions.

My curiosity about the quote source was related to Professional Concerns,
not in disagreement with John's post. Thanks, again, Karl

Dr. Karl I. Trappe Desk Phone: (512) 471-4152
Physics Dept, Mail Stop C-1600 Demo Office: (512) 471-5411
The University of Texas at Austin Home Phone: (512) 264-1616
Austin, Texas 78712-1081