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

on November 12 at 07:59PM

Validity of an argument

It is mentioned in the video how an argument is only valid if the conclusion follows logically from the stated premises, assuming those premises are true. Are we supposed to always assume the premises are true? For example, in the video, one of the arguments shown is the one with the two premises being "anyone named Sue is a girl" and "X is named Sue", and the conclusion is "X is a girl". How is this not a valid argument assuming the premises are true?

3 Replies

Irina on November 12 at 09:11PM

@mresende,

You are correct - this is a valid argument. It appears that the video uses the terms "valid" and "sound" interchangeably. The argument is valid if the conclusion follows logically from the premises assuming the premises are true, in other words, premises guarantee the conclusion IF the premises are true. Note that one or all of the premises could actually be false, and the argument could still be valid.

(1) anyone named sue is a girl
(2) x is named Sue
(3) Therefore, X is a girl.

This is a valid argument even though premise (1) is false - surely, there are dogs named Sue for example. If we assume that both premises are true, the conclusion follows logically.

Now, the argument is sound if and only if it is both valid and its premises are actually true. The above argument is valid but it is not sound because one of the premises is false.

Here is another example of an argument that is valid but unsound:

(1) All cats are dogs
(2) X is a cat
(3) Therefore, X is a dog

The validity only focuses on the logic/ form of the argument, not the substance.

Since the LSAT tests your logical reasoning rather than formal logic, the goal is to construct arguments that are both sound and valid, perhaps that is the reason the video uses these terms interchangeably.

Let me know if you have any other questions.

Dan Sunday at 05:05PM

Can we then say that example 2 IS VALID but NOT SOUND?

On the LSAT then, we do care about the soundness of an argument since we are to accept all premises as if they are true?

Skylar Monday at 01:12AM

@Dan-Engel Yes, that is correct. However, the LSAT is looking for arguments that are both sound and valid, so we care about both. Therefore, a false premise would lead to a flawed argument. Nevertheless, you are unlikely to see this on the LSAT as a false premise is too straightforward for the logic that the exam is looking to test, which is why the video focuses on evaluating if conclusions necessarily follow from the given premises.

Hope this helps and best of luck with your studies!