Common LSAT Logical Fallacies: Composition

Have you ever heard that inane little riddle, “Which weighs more, a ton of bricks or a ton of feathers?”

It’s gotta be bricks, they’re way heavier. Right?

The crux of this pissant’s play is the “ton,” of course; we’ve already established that their weights are equivalent. Thus, neither weighs more than the other. So why does anybody ever fall for something so silly?

We fall for it because we’re seduced by the Composition Fallacy. We believe, erroneously, that the things the ton is composed of has bearing on the weight of the whole. Such part-to-whole reasoning is not justified, for the same reason that you can’t make assumptions about a whole population based on the small subsection of people you’ve met.

The Composition Fallacy is a staple of the LSAT. You might see:

“If Bill stands up at the basketball game, then he can see better. Thus, if everyone stood up at the basketball game, everyone could see better.”

Or perhaps:

“Neither of the knives I tried were helpful in cutting my coconut. So no knives will help me cut my coconut.”

In both cases, we take knowledge about a part and invalidly infer information about the whole. In neither case does our inference hold; we’d be left with sore legs, a poor view of the game, and an unopened coconut if we didn’t know better than to reject the Composition Fallacy.

In addition to this part-to-whole logical misstep, we should be wary of the reverse – a whole-to-part relationship. For example:

“Our fourth grade class is innovative and kind.”

This statement does not allow you to infer that each individual student is innovative and kind, just because the whole has these two attributes. Gregory, for example, is in the fourth grade class but is dumb as a doornail (even his mother knows it) and just pulled Sally’s pigtails. Thus, Gregory demonstrates the illegitimacy of inferring that parts of the whole necessarily share the same attributes as the whole.

There is, unfortunately, no better way to illustrate this fallacy than with the following let-down: Gather some of your favorite stars — let’s say Ashton Kutcher, Topher Grace, Jennifer Garner, Jamie Foxx, Jessica Alba (necessarily), Bradley Cooper, Anne Hathaway  — and it looks like we’re in for a fantastic movie.

We’re not. This is the cast of Valentine’s Day, which is rated 18% on Rotten Tomatoes. No amount of Ashton Kutcher wearing stupid hats can make that number any better. Surmising that the viewers (and the reviewers) were mostly self-selecting – probably high schoolers at a sleepover – we might reasonably suspect that that 18% would drop still further if we sampled the general population.

What does this tell us? Despite having great parts, we have a bad movie, which illustrates how inferences made about a whole from its parts are potentially erroneous, and therefore invalid and fallacious.

The takeaway: don’t fall for either part-to-whole or whole-to-part fallacies on the LSAT. They’re easy to spot and should be guaranteed points on your Flaw questions. Equally importantly, don’t stand at basketball games, don’t see Valentine’s Day, and don’t let Gregory get near your pigtails.

For more practice on the Composition Fallacy, check out the LSAT questions below.

PrepTest 19 (June 1996 LSAT), S4, Q3 (p36); PrepTest 31 (June 2002 LSAT), S3, Q18 (p100); PrepTest 35 (October 2001 LSAT), S4, S18 (p245).

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