June 2010 LSAT Section 5 Question 9

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

3 Replies

Irina on November 6 at 04:08PM

@16jsingramIt 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.

JuanCarlos on November 27 at 08:23PM

The logic is sound but the argument is invalid? So the answer on the LSAT would be that it is a flawed argument?Skylar on December 2 at 01:01AM

@jccc You are correct, as the LSAT is looking for arguments that are both sound and valid. Therefore, a false premise would lead to a flawed argument. However, 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!