Most Supported By Questions

[Administrator]

We recently received the following question from a student. An instructor will respond below.

I have a somewhat subtle question about a subset of Most Supported By questions.

Some Most Supported By questions provide a set of pieces of evidence, and they ask me to, effectively, deduce what that evidence shows. Those are fine. Others set up conditional chains, and ask me to make an inference supported by that chain. Those are fine too.

Every now and then, though, I am given a Most Supported By question where I am supplied with a premise (or premises) and a conclusion, and I am asked to, basically, infer the missing premise, which is supposedly supported by the remaining premise(s) and the conclusion. These sometimes feel to me more like assumption questions than Must Be True questions, and I wonder whether it is deductively valid to deduce any particular missing premise, given a premise and a conclusion. I'm doing fine on them, but they give me pause, so I thought I would run my thoughts/concerns about them by you.

For example, consider this argument:

Premise: Socrates is a cat
Conclusion: Socrates is mortal
Inferred premise (Most supported): All cats are mortal.

However, aren't there a bunch of different premises you could infer? For instance, "Some cats are mortal, and Socrates happens to be the variety of cat that is mortal?"

I sometimes find that some of the LSAT questions operate in this manner.

For instance, take #5 in section 3 of Preptest 55:

Premise: The star-nosed mole has a nose that ends in a pair of several-pointed stars, or tentacles that are crucial for hunting, as moles are poor-sighted.
Premise: These tentacles contain receptors that detect electric fields produced by other animals.
Conclusion: [These tentacles enable] the moles to detect and catch suitable prey such as worms and insects.
Inferred premise/most supported: Both worms and insects produce electric fields.

For example, here, couldn't you also have the inferred premise be: "The tentacles also pick up low-frequency sounds emitted by worms and insects?" (For what it's worth, this wasn't an option, and the only answer choice that contained a reasonable missing premise was the correct answer.)

I suppose my question boils down to this:

Am I correct that LSAC sometimes considers a reasonable missing premise/assumption of an argument to be "supported by" the remaining premises plus the conclusion, even though the missing premise may not be strictly logically entailed by them, as the Socrates example and mole example may show?

[James Finch]

Hi,

A Most Strongly Supported question is asking for a likely inference, not necessarily a new conclusion. This gives the test makers the leeway to ask about assumptions being made as well, as if we accept everything in the stimulus as true (including any conclusions) the assumptions will have textual support. In the question you cite, as you note the only way that the tentacles can enable the moles to detect suitable prey is if that prey produces electric fields, so the only way for the conclusion to be true is if the missing assumption is true as well. So answer choice (A) is a necessary assumption to the argument; if we take the conclusion as true (sort of a reverse Justify question) then (A) must be true as well, and thus it is clearly the most strongly supported of the answer choices.

The example you gave of another possible assumption (the tentacles detect sounds) has no textual support in the stimulus and thus cannot be considered as an answer choice for a MSS question type. Even on an Assumption question, this would normally be presented on the LSAT as two separate premises, that the tentacles detect sounds and that the prey make detectable sounds. In everyday writing it would be more efficient to write this information together, but the LSAT is concerned with how test takers are able to deal with pertinent information/premises and structure of argumentation, so they will be asking for a single assumption rather than two combined assumptions as you've written. It would be much more likely to see one (or both) of those pieces of information given out separately as an incorrect choice, as neither one would be necessary to the argument given in the stimulus by itself.

Your Socrates example presents the same problem, you've combined two separate assumptions into one sentence. However, the LSAT still considers that two separate pieces of information, and thus two separate assumptions, similar to how different clauses in a sentence may be different elements of argumentation, i.e. the statement "Since I went to the store I have the ingredients to make dinner tonight" contains first a premise then a conclusion. Similarly, in the Socrates example, your hypothetical answer introduces an unnecessary step that won't be present in a correct answer choice:

Premise: Socrates is a cat
Conclusion: Thus Socrates is mortal
Inferred premise (Most strongly supported): All cats are mortal.
Incorrect answer choice 1: Some cats are mortal (what about Socrates?)
Incorrect answer choice 2: Socrates is a mortal cat-type (circular reasoning, the conclusion already says this)

The last issue here is that the conclusion that Socrates is mortal would almost always be given on the LSAT as a premise, and thus we could conclude that some cats are most likely mortal based upon Socrates's mortality. And this is how most of the Most Strongly Supported questions are structured.

Most important to take away from this is that only one of the answer choices will ever be supported at all by the stimulus, regardless of whether it is an assumption that we can infer or a conclusory inference. None of the other answer choices in this question have any textual support in the stimulus, so we're left with (A).

Hope this helps!