💡Tired of Studying?There's a shortcut.
Hire an expert to take your LSAT!Validity and Must Be True Questions
Validity
HERE’S THE SIMPLE WAY TO UNDERSTAND VALIDITY.
Would you agree with me that “All Jedi use the Force?”
Would you further agree with me that “Luke Skywalker is a Jedi?”
Then, you would agree with me that “Therefore, Luke Skywalker uses the Force.”
That right there probably represents about a quarter of all valid arguments on the LSAT. The other three-fourths we’ll get to shortly, but let’s first discuss what “validity” means.
Validity: Pretend that all the premises are true. Then, ask if the conclusion must also be true. If the answer is “yes,” then the argument is valid. If the answer is “no,” then the argument is not valid.
That’s how you figure out whether an argument is valid or not. Let’s apply the definition to our argument, rearranged below.
Premise 1:
All Jedi use the Force.
Premise 2:
Luke Skywalker is a Jedi.
Conclusion:
Therefore, Luke Skywalker uses the Force.
Let’s pretend that both premises are true. Easy, they are. Must the conclusion then also be true? Yes, absolutely. Can you even conceive of a world where simultaneously Luke Skywalker is a Jedi and all Jedi use the Force, but Luke doesn’t use the Force? No, you can’t. Because it’s nonsense. Great, so now you know what the relationship of validity looks like.
Remember when we first brought up the notion of validity in our discussion of what makes for a good argument? Well, there, we said that the best arguments, the ones with the strongest support relationship possible between the premises and the conclusion, the ones where there are absolutely zero assumptions being made, those were called the “valid” arguments.
Now test your understanding. Consider the argument below
Premise 1:
All gerbils are yeoogh.
Premise 2:
Darth Vader is a gerbil.
Conclusion:
Therefore, Darth Vader is yeoogh.
First, are any of these sentences true? No! All three sentences are false sentences. I didn’t even use real words for two of them. Now, is the argument valid? Apply the definition of validity, you can do it. Pretend that all the premises are true. Okay, we’re in pretend world where Darth Vader is a gerbil and all gerbils are yeoogh, whatever that is. Now, in our pretend world, must the conclusion then also be true? Sure enough, you are compelled by the force of logic to conclude that Darth Vader is yeoogh. Three false sentences make for a valid argument just as three true sentences did. We’ll learn why that is in another lesson.
LET’S REVIEW
Validity is a very special relationship between premises and the conclusion. Validity is the strongest of the support relationships. An argument which makes zero assumptions is valid.
The definition of a valid argument:
If the premises are true, then the conclusion must also be true.
Featured image: Matador-attribution-Wolfgang Staudt
Truth v. Validity
TRUTH IS NOT VALIDITY.
They are two concepts that are as different from each other as football is from origami. Truth and validity are not the same. You should never, ever, confuse the two.
Truth is a property of sentences (or to be more precise, declarative statements). I think we all know the definition of truth and yes, it’s what you think. For a statement like “all dogs go to heaven,” it’s true if all dogs go to heaven. It’s false when it’s not the case that all dogs go to heaven. False statements are sometimes called lies.
Let’s bring the distinction home. Validity is a property of arguments. Validity is not a property of statements. Truth is a property of statements. Truth is not a property of arguments. What does this mean? Try thinking about this example. I think we all know that we can’t say about the number "2" that it’s happy. Why? Because it just doesn’t make sense. Why? Because emotional states are not properties of numbers. Analogously, you can’t say about an argument that it’s true or false. Simply because truth isn’t a property of arguments.
I know this may come off as a little counter-intuitive at first, but that’s because you’re not used to the concepts of "validity" and “arguments” whereas you’re very familiar with emotions and numbers. But let me assure you that it’ll take no longer than… um… a month or two of hard thinking to become fluent with these concepts. Sorry, but that’s the way it is. As for validity, again, look at the definition: if (or pretend that) all the premises are true and when that’s the case, then the conclusion must also be true. See how it would make no sense to talk about whether a sentence is valid or not? Just like it makes no sense to say whether dogs are even or odd. Numbers are even or odd. Dogs are awesome.
LET’S REVIEW
Truth is a property of sentences. Sentences are true or false. Sentences are not valid or invalid. Validity is a property of arguments. Arguments are valid or invalid. Arguments are not true or false.
Featured image: Pink Elephant-attribution- MsCaprikell
Valid Argument Forms
Valid Argument Form (1 of 9)
Valid Argument Form 1 - Review
Valid Argument Form (2 of 9)
Valid Argument Form 2 - Review
Valid Argument Form (3 of 9)
Valid Argument Form 3 - Review
How to Approach Must be True Questions
INFERENCE: MUST BE TRUE (MBT) QUESTIONS.
The majority of these questions use formal logic, i.e., they contain conditional or intersection statements (which we will learn about in future lessons). If you train hard in translating from English to Lawgic, you’ll be able to solve these questions like math problems. If you’re aiming to score above a 170, you ought to consider these questions freebies.
Your understanding of validity is tested here. For these MBT questions, you are asked to select an answer choice such that when the statements in the stimulus are true, the right answer choice must also be true. In other words, your task is to formulate a valid argument.
Very much like the MSS questions, the MBT questions also take information from the stimulus and forces it down into the answer choices to prove one of them correct. The standard of proof is higher for these questions than for MSS questions.
Doing these questions well will improve your ability to do Sufficient Assumption questions.
SOME SAMPLE MBT QUESTION STEMS
- If the statements above are true, which one of the following must be true?
- Which one of the following can be properly inferred from the statements above?
- Which one of the following statements follows logically from the statements above?
LET’S REVIEW
MBT questions tend to make heavy use of logic. Your ability to make inferences is tested.