Not "leads to," jdavidwik - that's causal language, not conditional language. Nitpicky of me, perhaps, but it's important to think about those two as distinct from each other, and using the right language helps you do that.
Conditionally, if all A's are B's, then it must be the case that some B's are A's. For example, if every pizza has mozzarella cheese on it, then some things with mozzarella cheese must be pizzas. We can do a "some" arrow that goes in both directions, even though we cannot do a conditional reversal of "if mozzarella, then pizza," as that would be a Mistaken Negation.
So let's build a chain here:
If pizza, then mozzarella
If mozzarella, then dairy
If dairy, then fattening
(Not that any of these must be true in the real world, mind you. Let's just accept these as true for our purposes here.)
Putting this together, we get:
We could now conclude that every pizza has something fattening on it. But we can also conclude that SOME things that are fattening are found on pizza! Not everything fattening - no ice cream on pizza, thank goodness - but at least some things that are fattening are dairy products, and some of those are mozzarella, and some of that is on pizza.
For the last examples in Kelsey's post, if all A's are B's, then it cannot be true that some A's are not B's, and it cannot be true that some things that are not B's are A's (because the A's all have to be B's). However, it could be true that some B's are not A's.
"Some" is a two-way street, while "all" and "most" only go one way. I hope that clears things up!
Adam M. Tyson
PowerScore LSAT, GRE, ACT and SAT Instructor
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