Thanks Avi, for some reason I just wasn't getting it. I think I was confusing myself by reading the premises as "all S in all circumstances is what P is in some circumstances". I was looking at the quantity and not the modality. Then I drew a diagram to represent the circumstances "surrounding" the terms. Being more of a visual thinker, the invalidity then became clear.

But still, some of these modal syllogisms aren't at all obvious.

In fig. 1, if you tried converting the minor premise, you would have immediately seen that while S was distributed, M was not. All S must be M converts to Some M can be S. Remember All S means: Each S - it is distributive, not collective or collectional. So, the all circumstances are all circumstances concerning each S, which are not the same set of circumstances... The universal proposition is a mere summary formula.

Note additionally that in the past, some logicians thought that a universal potential major premise in the first fig. could yield a valid conclusion. They did not realize the issues involved. So, you are in good company!

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