"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.