Logic is primarily concerned, not with the truth of propositions, but with the validity of inferences; and it has long been a commonplace of traditional logic that it makes no difference to the validity of an inference whether its premisses and conclusion are true or whether they are false. The argument is valid if the conclusion follows from the premisses, whether true or false, or, we may add, neither. It is true that we often say that if the premisses are true, then the conclusion is true. But this is a concession to the indicative mood which we need not make. In our terminology, we could ignore the dictors, and say that if the descriptors of the premisses describe a state of affairs, then the conclusion describes, at least partially, the same state of affairs. Whether the state of affairs is actually the case makes no difference to the validity of the argument. References to truth and falsehood are therefore irrelevant.
(R. M. Hare, "Imperative Sentences," chap. 1 in his Practical Inferences, New Studies in Practical Philosophy, ed. W. D. Hudson [Berkeley and Los Angeles: University of California Press, 1972], 1-24, at 18 [essay first published in 1949] [italics in original])