Conditional Reasoning
Before we start solving real LSAT questions, we need to work on some foundational skills. This one is dedicated to perhaps the most important skill you'll need in LR sections: diagramming conditional reasoning.
Let's start with what "conditional reasoning" actually means.
If/Then Statements
Any statement that can be boiled down to a simple if/then structure employs conditional reasoning. Here's a basic example:
If I go to Harvard Law, then I would have taken the LSAT.
Hopefully this just makes sense — of course someone would have taken the LSAT if they're going to Harvard Law. Even the charming Elle Woods had to take it.
Now let's analyze this if/then statement critically by breaking it into its two parts.
The IF — Sufficient Condition
The IF part of any if/then statement is called the sufficient condition. In our example, "I go to Harvard Law" is the sufficient condition.
If the sufficient condition is TRUE, the "then" part is instantly triggered and is also TRUE. In our example: if it's true that you go to Harvard Law, we can say with 100% confidence that you would have taken the LSAT. We don't need anyone to confirm it — because the sufficient condition being satisfied guarantees the "then" part.
Sufficient condition is TRUE "then" part is TRUE.
But what if the sufficient condition is FALSE — meaning you did not go to Harvard Law? Can we conclude anything about whether you took the LSAT? No. You could have taken the LSAT and gotten a weak score. You could have taken it and chosen a different school. You might not have taken it at all. We simply cannot infer anything from the fact that someone does not go to Harvard Law.
Sufficient condition is FALSE no conclusion possible.
The THEN — Necessary Condition
Now let's look at the THEN part, which is called the necessary condition. In our example, "I would have taken the LSAT" is the necessary condition.
If the necessary condition is TRUE — say, I tell you I've taken the LSAT — does that tell you anything about whether I got into Harvard Law? No. I could have gotten a great score but chose not to pursue law, or a bad score such that I did not get into Harvard Law, or anything in between. The necessary condition being true tells you nothing about the sufficient condition.
Necessary condition is TRUE no conclusion possible.
But what if the necessary condition is FALSE? If I tell you I have not taken the LSAT, does that tell you anything about Harvard Law? Yes — it tells you I definitely did not go to Harvard Law, because I haven't even met the basic requirement.
This is why the "then" part is called the necessary condition. It is a requirement for the sufficient condition to be true. If the necessary condition isn't satisfied, the sufficient condition can't be either.
Necessary condition is FALSE sufficient condition is FALSE.
Summary Table
| Case | What is given | What can be concluded |
|---|---|---|
| Sufficient condition is TRUE | "If" part is TRUE (I got into Harvard Law) | "Then" part is TRUE (I took the LSAT) |
| Sufficient condition is FALSE | "If" part is FALSE (I did not get into Harvard Law) | No conclusion about the "then" part (no idea if you took the LSAT or not) |
| Necessary condition is TRUE | "Then" is TRUE (I took the LSAT) | No conclusion about the "if" part (could have gotten a bad score, or been rejected for other reasons) |
| Necessary condition is FALSE | "Then" part is FALSE (I did not take the LSAT) | "If" part is FALSE (I did not get into Harvard Law) |
Beyond If/Then
Because English is an unnecessarily complicated language, "if" is not the only keyword for sufficient conditions, and "then" is not the only keyword for necessary conditions.
Sufficient condition keywords:
| Keyword / Phrase | Example |
|---|---|
| If | If I go to Harvard Law, then I have taken the LSAT. |
| All | All students who go to Harvard Law have taken the LSAT. |
| Every | Every student who goes to Harvard Law has taken the LSAT. |
| Any | Any student who goes to Harvard Law has taken the LSAT. |
| In the event that | In the event that someone goes to Harvard Law, they have taken the LSAT. |
| As long as | As long as someone goes to Harvard Law, they have taken the LSAT. |
| Each | Each student who goes to Harvard Law has taken the LSAT. |
| In order to | In order to go to Harvard Law, you need to take the LSAT. |
| When | When someone goes to Harvard Law, they have taken the LSAT. |
| Whenever | Whenever someone goes to Harvard Law, they have taken the LSAT. |
Not every statement will have the strict "if/then" structure, but they're all saying the same thing logically. Notice how all of these keywords are open and inclusive — that's the common thread. This list isn't exhaustive, but it covers the most common ones, and recognizing that property is more useful than memorizing the list.
Necessary condition keywords:
| Keyword / Phrase | Example |
|---|---|
| Then | If I go to Harvard Law, then I would have taken the LSAT. |
| Only if | I would go to Harvard Law only if I had taken the LSAT. |
| Only when | I would go to Harvard Law only when I had taken the LSAT. |
| Requires | Going to Harvard Law requires taking the LSAT. |
| Depends on | Going to Harvard Law depends on taking the LSAT. |
| Necessary | Taking the LSAT is necessary to go to Harvard Law. |
| Have to | I would have to take the LSAT to go to Harvard Law. |
| Needs | I need to take the LSAT if I want to go to Harvard Law. |
| Must | One must take the LSAT to go to Harvard Law. |
Again, no need to memorize this list. The common pattern here is that these words are restrictive rather than open — "needs to," "must have," "only if" — and that's the property you can use to spot necessary conditions.
Order Doesn't Matter
In our running example, the sufficient condition appeared on the left and the necessary condition on the right. But that's not always the case.
Consider the same example, flipped:
Only if I have taken the LSAT, would I be going to Harvard Law.
This is saying the exact same thing as before — but if you assumed the sufficient condition is always on the left, you'd misread it. Here, "only if" signals a necessary condition, even though it comes first.
This is what makes conditional reasoning tricky: multiple keywords, no consistent order. That's exactly why we use diagramming.
Diagramming
Diagramming is a way to simplify any conditional statement regardless of how it's worded. The format is simple:
A B — where A is the sufficient condition and B is the necessary condition.
Let's apply this to our example:
If I go to Harvard Law, then I would have taken the LSAT.
We strip away the identifying keywords and place the conditions in their correct positions:
Harvard Law Took the LSAT
Now, regardless of how the statement is phrased, it always reduces to the same diagram:
Only if I have taken the LSAT, would I be going to Harvard Law.
Harvard Law Took the LSAT ✓
I would have taken the LSAT in order to go to Harvard Law.
Harvard Law Took the LSAT ✓
The diagram is powerful because it makes the logic visual. If the sufficient condition is true, just follow the arrow — whatever it points to is also true. And if you only know the necessary condition is true, there's no arrow to follow back, which is exactly why you can't conclude anything about the sufficient condition.
A note on shorthand: Many prep materials suggest abbreviating your diagrams. For example, writing HL T LSAT where HL stands for Harvard Law and T LSAT stands for took the LSAT. This never worked for me personally. I would constantly go back to the question to remember what my shorthand meant, which gets especially painful once you have a lot of diagrams in a single question. Brief phrases worked better for me. That said, give abbreviating a shot if you want. Most people I know do it and it may work better for you. For clarity, I will use phrases throughout these notes.
AND / OR
AND
Consider this statement:
If I study consistently and I study smart, then I will get a good score on the LSAT.
This has a familiar if-then structure, but with a magic word in the middle: AND. Diagramming gives us:
Consistent studying AND Smart studying Good score on the LSAT
When AND appears in the sufficient condition, all parts must be TRUE for the sufficient condition to be satisfied. If only one is met (say you study smart but not consistently), the sufficient condition has not been triggered, and we cannot infer anything about the necessary condition.
Now consider AND appearing in the necessary condition instead:
If I go to Harvard Law, then I have a good GPA and I would have taken the LSAT.
Diagramming gives us:
Go to Harvard Law Good GPA AND Good LSAT score
Here, satisfying the sufficient condition (going to Harvard Law) automatically triggers all parts of the necessary condition. We can conclude that person has both a good GPA and a good LSAT score.
OR
OR is the counterpart to AND, and it works differently depending on which side of the arrow it appears on.
Consider this statement:
If I study consistently or I study smart, then I will get a good score on the LSAT.
Diagramming gives us:
Consistent studying OR Smart studying Good score on the LSAT
When OR appears in the sufficient condition, only one of the parts needs to be TRUE to trigger the necessary condition. Either studying consistently alone, or studying smart alone, is enough to conclude you will get a good score. You do not need both.
Now consider OR appearing in the necessary condition:
If I go to Harvard Law, then I have a good GPA or I would have taken the LSAT.
Diagramming gives us:
Go to Harvard Law Good GPA OR Good LSAT score
If the sufficient condition is satisfied (going to Harvard Law), we know at least one of the two necessary conditions is true. However, we cannot say with certainty which one it is. The person might have a good GPA but no LSAT score, or a good LSAT score but not a stellar GPA, or both. All we know is that at least one of them holds.
Here is the summary of AND/OR. Try and get an intuitive understanding of this concept instead of memorizing it.
| Position | AND | OR |
|---|---|---|
| Sufficient condition | All parts must be TRUE to trigger the necessary condition | Only one part needs to be TRUE to trigger the necessary condition |
| Necessary condition | Satisfying the sufficient condition triggers all parts | Satisfying the sufficient condition guarantees at least one part (but not necessarily both) |
The Contrapositive
Every conditional statement can also be expressed in its contrapositive form. The rule is:
A B is equivalent to: Not B Not A
These two diagrams say the exact same thing.
Applying this to our example:
Harvard Law Took the LSAT
Contrapositive:
Did NOT take the LSAT Did NOT go to Harvard Law
This is how we capture the last row of the summary table diagrammatically: if the necessary condition is false, the sufficient condition must also be false. The contrapositive makes that explicit.
Contrapositives come up a lot on the LSAT. This is one of the few things I'll ask you to actually memorize:
A B is the same as ¬B ¬A
(¬) is just short for NOT. Use it or don't use it, whatever works best for you.
What's Next
That was a lot — especially if you've never seen conditional logic before. Here's what I'd recommend:
First, breathe. Go touch some grass.
When you come back, review your notes if you made any, or skim this chapter again to reload the key ideas. Then, do the conditional reasoning drill to check your understanding.
Only move on once you can comfortably pass the drill. The next chapter is conditional reasoning on steroids — going in without these fundamentals will hurt you.