Which one of the following statements most accurately characterizes a difference between the two passages?

shunhe on January 9, 2021

Hi @Sarah-Michelle-Olivo,

Thanks for the question! So the answer to this question is surprisingly irrelevant for the LSAT, because on the LSAT, remember, you assume that all the premises are true. So you won’t run into the problem of whether or not an argument is “invalid” because of false premises, since the premises on the LSAT are never false.

That’s the short answer. For a more technical explanation (that, again, you don’t need to know for the LSAT), whatever example you saw was technically wrong. The definition of invalidity you posted is correct. Validity is basically just about whether or not the internal logic of the argument is correct. There’s another concept called “soundness.” An argument is sound if and only if it is valid and all its premises are true. So an argument can be valid, but not sound, if its internal logic makes sense but its premises are false. For example:

P: Unicorns exist.
P: If unicorns exist, I am Hercules.
C: I am Hercules.

Both of these premises are patently false. However, the internal logic is simply

P: A
P: A —> B
C: B

which is correct logic. So it’s a valid argument. Technically, what the example should’ve said was that the argument was unsound (or I suppose “flawed” would work too), but valid. However, since “soundness” never is at play with the LSAT, whoever wrote the example probably felt justified in being slightly sloppy with their wording.

Hope this helps! Feel free to ask any other questions that you might have.

Sarah-Michelle-Olivo on January 15, 2021

Thank you

shunhe on January 16, 2021

Glad I could help! Let us know if you have any further questions.