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

Logan on January 22, 2020

Valid Arguments

To be clear, on p. 6, the second example is logically valid--that is, regardless of truth in the real world, structurally the argument is valid--but it is not sound or true? Am I correct that in formal logic the word "valid" simply refers to the structure of some argument, not the truth value? I want to be sure that I am not confused about this. I am aware that truth value does matter on the LSAT, and therefore this argument would not be sound because some boy could be named Sue. However, the arguments structure is still valid, correct?


on January 22, 2020

Hello @logan997,

Great question, this is an important step for entering the LSAT world. Sometimes we have to leave our knowledge of the real world behind. In the real world, it is probably not true that anyone named Sue is a girl. There are probably some boys named Sue. However, when a question gives us a premise, we are to accept it as fact.

Your concept of a valid argument structure is correct. This is the only thing we care about. We do not care about real-world truth. I'll give you another example.

Premise: All fish lay eggs.
Premise: This animal does not lay eggs.
Conclusion: This animal is not a fish.

This is a valid argument if we accept the truth of the premises (which we must do).

Let's say that I am aware of the fact that some sharks give live birth. Does this make the argument invalid? No. It is not my job to evaluate the truth of the premises. I'm going to forget that real-world knowledge and I'm going to enter the reality of this question.

You're going to come across questions that have ridiculous sounding premises, but you have to take them seriously. Obviously, there are some assumptions/universal truths that we are allowed to bring to the test. For example, humans breathe oxygen, gravity exists, and getting a disease is a bad thing. (If a conclusion says that a certain medication is beneficial, we aren't going to say, "Invalid argument. What if some people wanted to be sick?")

The LSAT isn't asking you to abandon all understanding of reality. However, when we are given a premise, we must accept it as true even if it contradicts something we know.

Sarah on February 8, 2020

Hi I have a follow up question. Why is the argument you are referring to considered a flawed argument like it says in the video? If we follow the premises logically then it should be a valid argument right? Or is it a valid argument with unsound premises?

Joti on June 2, 2021

^^ I was also wondering the same thing, why is the second argument invalid if we don't hear the song of if the question regarding that argument was "Which of the following would weaken this argument" and then we could possibly find an answer that says "A man is named Sue" to make it invalid because we proved a premise to be wrong. However, the instructor for the video doesn't mention that the argument would be wrong in that manner, just simply playing the song (bringing in outside information) to say it is invalid. I thought we were expected to only use the information given in the question/passage?