“That’s all very well in theory, but it won’t do in practice.” In this sophism you admit the premisses but deny the conclusion, in contradiction with a well-known rule of logic. The assertion is based upon an impossibility: what is right in theory must work in practice; and if it does not, there is a mistake in the theory; something has been overlooked and not allowed for; and, consequently, what is wrong in practice is wrong in theory too.

From The Art of Controversy