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In a recent issue of “The New Yorker” magazine, the following excerpt from the “Philadelphia Welcomat” was reprinted:
If Leonardo da Vinci had been born a female the ceiling of the Sistine Chapel might never have been painted.

“The New Yorker” commented:
And if Michelangelo had been Siamese twins, the work would have been completed in half the time.

The point of “The New Yorker”’s comment is not that such counterfactuals are false; it is more that anyone who would entertain such an idea — anyone who would “slip” the sex or number of a given human being — would have to be a little loony. Ironically, though, in the same issue, the following sentence, concluding a book review, was printed without blushing:
I think professor Philipp Frank would have enjoyed both of these books enormously.

Now poor Professor Frank is dead; and clearly it is nonsense to suggest that someone could read books written after his death. So why wasn’t this serious sentence also scoffed at? Somehow, in some difficult-to-pin-down sense, the parameters slipped in this sentence do not violate our sense of “possibility” as much as in the earlier examples.

Something allows us to imagine “all other things being equal” better in this one than in the others. But why? What is it about the way we classify events and people that makes us know deep down what is “sensible” to slip, and what is “silly”?