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February 27, 2022

What makes an argument valid or invalid? Why is validity important on Logical Reasoning? Learn the differences between good and bad arguments to improve your LSAT score.

A valid argument provides all the information needed to prove its conclusion. In a valid argument, if the premises are true, the conclusion must be true as well.

Examples of Valid Arguments

Some valid arguments are more intuitively valid than others. Here’s a valid argument that you probably have no problem accepting:

1. Ralph is a dog.
2. No dogs are allowed on the roller-coaster.
3. Therefore, Ralph is not allowed on the roller-coaster.

The following argument, though counterfactual, is also 100% valid:

1. Every dog is a reptile.
2. Every reptile is cold-blooded.
3. Therefore, every dog is cold-blooded.

Each of the arguments above is valid because the truth of its premises guarantees the truth of its conclusion.

Of course, in the second example above, the premises are false. But that doesn’t mean it’s an invalid argument. On the LSAT, your job isn’t to argue with the premises. Your job is to accept the premises and to object when those premises don’t prove the conclusion. Read more about attacking the argument here[Accept the Premises but Attack the Conclusion].

Invalid Arguments

An invalid (i.e. flawed) argument is one whose conclusion is not proven by its premises. That is, even if all the premises are true, the conclusion could still be false. Some sort of jump in reasoning has taken place, and it’s your job to figure out where the argument went wrong.

Examples of Invalid Arguments

Consider the following argument:

1. Being friendly is the easiest way to make friends quickly.
2. Alana has a lot of friends.
3. Therefore, Alana must be very friendly.

The conclusion above is not proven by the premises. The argument tells us that being friendly is one way to make friends, but is that the only way? And does having a lot of friends necessarily mean that you are very friendly? Although Alana might be very friendly, the author hasn’t proven that she is.

Why is Validity Important on the LSAT?

Both valid and invalid arguments appear on Logical Reasoning sections. The best strategy to use on a given question depends on whether the argument you’re dealing with is valid or invalid.

Identifying Arguments

The first step is to determine whether or not the passage is an argument at all [LR Tips: Arguments & Indicators]. If it is an argument, your next step is to determine whether the argument is valid or invalid. Identify the conclusion and the evidence presented in support of that conclusion. Then ask yourself: Is the conclusion proven by that evidence? Often, the author thinks they have proven their conclusion, but they actually haven’t. Don’t take the author’s word for it. 

If the argument is valid, you can’t argue with it. If it’s invalid, you must argue with it.

Understanding & Pointing Out Logical Flaws

“Arguing” with the argument really just means pointing out its logical flaws. The conclusion may be too broad to be proven by the premises. The conclusion may require an unwarranted assumption. The author may have sneakily shifted their definition of a certain word halfway through the argument. The author may try to pass off evidence of a correlation as proof of a causal relationship. 

There are endless ways an argument can go south, because there are infinite ways to be stupid. On logical reasoning questions, your job is to figure out exactly where the author went wrong. Learn more about the most common flaws tested on LSAT Logical Reasoning here [Common Flaws].