The very same point, substantially, is raised in the discussion between Mr. John Stuart Mill and an opponent, in the note above referred to. "A writer in the 'British Quarterly Review' endeavours to show that there is no _petitio principii_ in the Syllogism, by denying that the proposition All men are mortal, asserts or assumes that Socrates is mortal. In support of this denial, he argues that we may, and in fact do, admit the general proposition without having particularly examined the case of Socrates, and even without knowing whether the individual so named is a man or something else. But this of course was never denied. That we can and do draw inferences concerning cases specifically unknown to us, is the datum from which all who discuss this subject must set out. The question is, in what terms the evidence or ground on which we draw these conclusions may best be designated--whether it is most correct to say that the unknown case is proved by known cases, or that it is proved by a general proposition including both sets of cases, the known and the unknown? I contend for the former mode of expression. I hold it an abuse of language to say, that the proof that Socrates is mortal, is that all men are mortal. Turn it in what way we will, this seems to me asserting that a thing is the proof of itself. Whoever pronounces the words, All men are mortal, has affirmed that Socrates is mortal, though he may never have heard of Socrates; for since Socrates, whether known to be a man or not, really is a man, he is included in the words, All men, and in every assertion of which they are the subject. . . . . The reviewer acknowledges that the maxim (Dictum de Omni et Nullo) as commonly expressed--'Whatever is true of a class is true of everything included in the class,' is a mere identical proposition, since the class _is_ nothing but the things included in it. But he thinks this defect would be cured by wording the maxim thus: 'Whatever is true of a class is true of everything which can be shown to be a member of the class:' as if a thing could be shown to be a member of the class without being one."