Some, most, all: the LSAT words that mean less than you think

Everyday English uses these words loosely, and the test does not. Most of the damage they do is not that students find them hard — it is that students do not notice them at all, because they read the sentence the way they would read it in conversation.

The definitions

• Some means at least one. That is the whole meaning. It is compatible with two, with half, and with all of them.
• Most means more than half. It is compatible with all of them.
• All means every one, with no exceptions.
• Few and a few are not the same in ordinary usage, and the test avoids relying on that distinction — but where it appears, treat it as at least one unless the passage says otherwise.

The first two are where the points are. In conversation, "some students passed" implies not all of them did. On the LSAT it implies no such thing, and an answer choice that assumes it does is wrong.

What conversation smuggles in

Normal speech carries an implication: if I say some of the cake is gone, you assume not all of it is, because otherwise I would have said all. That inference is a social convention, not logic, and the test does not honor it.

This is the source of an entire family of wrong answers. The stimulus says most; the tempting choice says most but not all. The stimulus says some; the choice says some but not others. Both add a restriction that was never stated, and both are attractive precisely because in ordinary conversation you would have been right to add it.

What follows and what does not

• Some X are Y does tell you some Y are X. The relation runs both ways.
• Most X are Y does NOT tell you most Y are X. Most surgeons are university graduates; most university graduates are not surgeons.
• Two "most" statements about the same group do combine: if most X are Y and most X are Z, then at least one thing is both Y and Z — because two groups each larger than half of X have to overlap.
• Two "some" statements combine into nothing. Some X are Y and some X are Z tells you they might overlap, and might not.

That third one is the only genuinely non-obvious rule here, and it shows up on hard Inference questions. Two majorities of the same group cannot be disjoint. Everything else follows from reading the words literally.

How to use this under time

You do not need notation for most of it. What you need is to slow down for a quarter of a second when one of these words appears, and read it as its definition rather than as speech. "Some" — at least one. "Most" — more than half.

Then, when you are checking an answer choice, ask whether it has quietly strengthened the quantifier. Choices that turn some into most, or most into all, are among the most common wrong answers on the test, and they are invisible unless you are looking for them.