Here is an excerpt from Russell's fifth lecture on Logical Atomism:

"This notion, of course, of general propositions not involving existence is one which is not in the traditional doctrine of the syllogism. In the traditional doctrine of the syllogism, it was assumed that when you have such a statement as “All Greeks are men,” that implies that there are Greeks, and this produced fallacies. For instance, “All chimeras are animals, and all chimeras breathe flame, therefore some animals breathe flame.” This is a syllogism in Darapti, but that mood of the syllogism is fallacious, as this instance shows. That was a point, by the way, which had a certain historical interest, because it impeded Leibniz in his attempts to construct a mathematical logic. He was always engaged in trying to construct such a mathematical logic as we have now, or rather such a one as Boole constructed, and he was always failing because of his respect for Aristotle. Whenever he invented a really good system, as he did several times, it always brought out that such moods as Darapti are fallacious. If you say “All A is B and all A is C, therefore some B is C” – if you say this you incur a fallacy, but he could not bring himself to believe that it was fallacious, so he began again. That shows you that you should not have too much respect for distinguished men."

Related In logic, do propositions default to true or false when objects in them do not exist?

– Conifold – 2019-01-07T04:56:40.403I whatched the same video and I kept waiting a further explanation about darapti invalidity. Thanks Five Omega for your respond, it helps to identify the reason of the dispute. However, the utility is not shown yet. To me if some concept (like unicorn)does not refer to someting real, that is not a problem logic, as "rules of inference". In other words, fallowing your wxplanations, it seems that the concern on this matter would be about truth not about validity. It is hard to imagine a practical situation where darapti should not be consider as conclusive argument without checking the value of – None – 2016-10-05T14:43:14.647