The Difference Between Validity and Truth on the LSAT

What's the Difference Between Validity and Truth on the LSAT?
You might hear terms like “valid argument” and wonder what that really means. Does a valid argument automatically mean true? Not necessarily! Here is a guide to understanding the difference between validity and truth on the LSAT.

What is an Argument?

An argument is the combination of supporting statements that someone claims results in the truth of a conclusion. The truth of an argument’s supporting statements, or premises, should naturally lead to the truth of its conclusion.

Since we’re talking about validity, we’re only concerned with arguments in terms of “formal” logic. With formal logic, we aren’t dealing with uncertainty. You’re meant to reach a conclusion from the premises, step by step. The conclusion should necessarily follow from the premises.

For example, take a look at this formal argument. “If someone is shorter than 5ft., then they absolutely can’t ride this ride. Jane is shorter than 5ft. In conclusion, Jane can’t ride this ride.”

If the premises are true in this argument, we can be 100% sure that the conclusion is, too.

What is a Valid Argument?

Take a look again at the argument above. Is there any way to argue against it? If the two premises are true, it must be a fact that Jane can’t ride the ride. When you absolutely must accept the conclusion based on the premises, we can say the argument is “valid.” We can’t really add any premise to this argument to make it any better; the conclusion is already 100% locked in if the premises are true.

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If an argument’s conclusion must be true if its premises are true, then it is a valid argument.

What is a Invalid Argument?

We have only two choices when it comes to these arguments–they’re either valid or invalid. So, if an argument’s not valid for any reason, it must be invalid.

Take a look at the argument below. Consider, does the conclusion 100% have to follow from the premises?

Premise: If Dan won the Mega-Buck Jackpot tonight, he could be a millionaire.
Premise: Dan didn’t win the Mega-Buck Jackpot.
Conclusion: Dan can’t be a millionaire.

Does that really make sense? Let’s imagine that the premises are really true. Does that have to mean that Dan can’t be a millionaire? Absolutely not! There are many other ways Dan could become a millionaire. He could suddenly inherit a wealthy relative’s millions. He could find millions of dollars in the street and get to keep it. Or he might get a star role in an action movie!

An Argument Can be Valid with False Premises

Remember that validity and truth are very separate in logic. We can map out an air-tight argument, but with completely false support. Examples of this can be seen on crime shows very often. Let’s say someone is defending against accusations of theft in court. The suspect presents a great alibi:

Premise: The theft of the diamond occurred at 4:21 A.M. on April 8th, 2015 in Santa Barbara, California.
Premise: Mr. Johnny Ripoff was in Maine, teaching his niece how to knit, the evening of April 7th, 2015, and was peacefully sleeping in his sister’s home during the time of the theft.
Conclusion: Mr. Ripoff could not have committed this crime.

Sounds good, doesn’t it? How could someone be in two places at once? This could be considered a valid argument. You’d have to accept the conclusion if he really was in Maine. But, obviously, we have to consider whether or not he’s lying. Is there any actual proof that he was really in Maine? Can any reliable witnesses confirm that they saw him on the east coast? This, by the way, is why lawyers present evidence in court. Without evidence, how can we objectively see the truth? Imagine if court cases could be won on valid arguments alone! That would certainly make the lawyer’s job easier, but it definitely isn’t the case.

A Premise Can’t Be “Valid” and an Argument Can’t be “True”

Remember that statements, by themselves, can’t be called valid or invalid. Sure, in conversation, we might say “that’s valid” to our friends when we mean to say “that’s true.” But on the LSAT, we shouldn’t mix up language in this way. “Valid” describes the relationship between multiple statements, and truth/falsity only applies to individual premises.

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The reverse is also true. We can’t call an entire argument “true.”

What if the Argument is Valid AND All of its Premises are True?

If an argument is both valid and all of its premises are true, you can say that it’s a “sound” argument. This is clearly the ideal goal when making a sincere argument.

Should You Be Concerned with Truth on the LSAT?

When it comes to the arguments you’re going to come across in the Logical Reasoning section or Reading Comprehension, rest assured that you do not have to determine whether the premises are literally true. You can simply assume that the speakers in the questions are not lying. Even if they were lying, your main concern is not with the truth of their statements, but with the validity of the argument. Remember to ask yourself, “Could the conclusion be false even if all of the premises are true?” If this is the case, the argument is invalid.

I hope all of these examples have made clear what the difference is between validity and truth on the LSAT! To dive deeper into logical reasoning on the LSAT and the many other aspects of arguments you’ll need to know, remember to sign up for Magoosh Online LSAT Prep!


  • Deborah

    Deborah earned her undergraduate degree from Brown University in 2010 and MBA from Salve Regina University. She scored in the 96th percentile on the LSAT and loves finding better ways to understand logic and solid arguments. When she’s not teaching, she enjoys volunteering, reading adventure fiction, and adding tech skills to her toolbox.

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