Don't Mix Up the Reasoning and the Conclusion
"Since A, B." is different from "If A, then B."
Do these two arguments have the same structure?
If Sandra wanted to join us on the trip, she would have said so.
Since she didn’t say so, she clearly doesn’t want to join.
If Theron is taking the bus to get here, he’ll arrive within 3 hours.
So if he doesn’t arrive within 3 hours, that means he isn’t taking the bus.
Savvy LSAT students would realize that the answer is…no, they don’t. In the first argument, we have 2 premises:
(1) If S wanted to join, she would have said so.
(2) She didn’t say so.
Based on those premises, we reach the conclusion: She doesn’t want to join. The conclusion is *not* the entire statement beginning with “since” - it’s just the second half.
In the second argument, however, there’s only 1 premise:
(1) If T is taking the bus, he’ll arrive within 3 hours.
Then we get the conclusion, which is a conditional: “If he doesn’t arrive within 3 hours, he isn’t taking the bus.”
Even though the logic of the arguments is similar — they’re both using the contrapositive form of reasoning — the structure definitely is different in the eyes of the LSAT.
Too often students will think of a line that says “Since A, B” as one conditional-like statement “If A -> B”. But we have to keep the two halves of a sentence with “since” separate: the half connected with since is a line of support, and the other half is a conclusion. Even though the author would *agree* with the idea that if A is true, B is true, that idea is just part of the underlying reasoning for how the author gets to the conclusion of B.
If you don’t recognize the point described above, then you’ll sometimes find it impossible to distinguish between two answers on Parallel Reasoning questions.
You also might run into trouble on Main Conclusion questions.
For example, take this argument:
Some people say that we should pick Randall for our basketball team next week, since he’s unusually tall. But because today’s style of basketball game requires speed and agility more than height, Randall is not a good selection.
What’s the main conclusion of this argument?
(a) If today’s stye of basketball game requires speed and agility more than height, then Randall is not a good selection.
(b) Randall is not a good selection for our basketball team next week.
The answer is (b). (a) is definitely part of the reasoning for how the author gets to the conclusion, but it’s not the conclusion itself. It’s the assumption that links the premise *to* the conclusion.
Keep this idea in mind and you’ll find a decent number of Main Conclusion and Parallel Reasoning questions easier.
Future LSAT Reading Comp. Passages
This section of the post collects interesting online articles that you might see edited down to an actual LSAT RC passage in the distant future. Read these recommendations every week and I can guarantee that your RC score will improve or you'll learn something interesting about the world, or both.
Thanks for reading,