It depends on what the meaning of the word 'is' is.

[max_i_m]

“made from potato flour in the sense that one cannot say that it is not made from potato flour"

The Lord Justice Hath Ruled: Pringles Are Potato ChipsAs a mainstream mathematician, I'm pleased to see a court ruling utilizing proof by contradiction. The article goes on to state

Comments

[poslushnik]

> As a mainstream mathematician, I'm pleased to see a court ruling utilizing proof by contradiction.

Is there some weird sect of mathematicians who do not believe in proofs by contradiction?

[max_i_m]

http://en.wikipedia.org/wiki/Double_negative_elimination

Double negative elimination is a theorem of classical logic, but not intuitionistic logic. Because of the constructive flavor of intuitionistic logic, a statement such as "It's not the case that it's not raining" is weaker than "It's raining". The latter requires a proof of rain, whereas the former merely requires a proof that rain would not be contradictory. (This distinction also arises in natural language in the form of litotes.)

See also

http://en.wikipedia.org/wiki/Intuitionistic_logic

Disclamer: I know nothing about logic, classical or otherwise (well, ok I read Logic as Algebra at some point).

[poslushnik]

Ah, I've heard about something like that, except under some other name.

I'm curious how careful they are not to use proof by contradiction while proving things :)