The unduly modest David Brightly has begun a weblog entitled tillyandlola, "scribblings of no consequence." In a recent post he criticizes my analysis of the invalidity of the argument: Man is a species; Socrates is a man; ergo, Socrates is a species. I claimed that the argument equivocates on 'is.' In the major premise, 'is' expresses a relation of conceptual inclusion: the concept man includes the subconcept species. In the minor premise, however, the 'is' is the 'is' of predication: Socrates falls under man, he doesn't fall within it.
I am afraid that my analysis is faulty, however, and for the reasons that David gives. There is of course a difference between the 'is' of inclusion and the 'is' of predication. 'Man is an animal' expresses the inclusion of the concept animal within the concept man. 'Socrates is a man,' however, does something different: it expresses the fact that Socrates falls under the concept man.
But as David notes, it is not clear that species is included within the concept man. If we climb the tree of Porphyry we will ascend from man to mammal to animal; but nowhere in our ascent will we hit upon species.