Conditional logic shows up all over the LSAT. Words like if, only if, unless, and all create rules, and many Logical Reasoning questions test whether you read those rules correctly. Logic Games are gone, but conditional logic is still a big part of the test.
This guide explains the basic rules, how to translate tricky words, and the common traps. No diagrams are required, but they help.
The Basic Rule: If A, Then B
“If A, then B” means: whenever A happens, B must happen.
- A is the sufficient condition. It is enough to guarantee B.
- B is the necessary condition. It must be there whenever A is.
We write this as A → B.
Example: “If it rains, the game is canceled.” Rain → Canceled.
The Contrapositive: The One Valid Move
From A → B, you can always conclude not B → not A. This is called the contrapositive.
Example: “If it rains, the game is canceled.” So: if the game is not canceled, it did not rain.
To make a contrapositive: flip the order and negate both sides.
The Two Big Traps
| Trap | Form | Why It Is Wrong |
|---|---|---|
| Mistaken reversal | A → B, so B → A | The game could be canceled for another reason, like snow. |
| Mistaken negation | A → B, so not A → not B | No rain does not mean the game happens. Something else could cancel it. |
Many wrong answers on flaw questions and parallel reasoning questions use these two mistakes.
How to Translate Conditional Words
| Phrase | Translation |
|---|---|
| If A, then B | A → B |
| All A are B | A → B |
| Every A is B / Any A is B | A → B |
| A only if B | A → B |
| Only A are B | B → A |
| A requires B / A needs B | A → B |
| No A are B | A → not B |
| A unless B | not B → A |
| A if and only if B | A → B and B → A |
“Only If” Is the Most Missed
“A only if B” means B is necessary for A. So A → B. The word after “only if” goes on the right side of the arrow.
Example: “You can enter only if you have a ticket.” Enter → Ticket. Having a ticket does not guarantee you get in, but you cannot get in without one.
“Only” vs “Only If”
“Only A are B” flips it: B → A. “Only members can vote” means Vote → Member.
How to Handle “Unless” (A Unless B, X Unless Y)
“Unless” is the word students fear most, but it has a simple rule:
- Negate the part that is not attached to “unless.”
- Make that the sufficient condition.
- The “unless” part becomes the necessary condition.
So “A unless B” becomes not A → B. You can also write it as not B → A. Both are correct, because one is the contrapositive of the other.
Example: “The picnic will happen unless it storms.” Not storm → Picnic. Or: No picnic → Storm.
Watch out: “Unless” does not tell you what happens if it does storm. The picnic might still happen in a tent.
Chaining Rules Together
If A → B and B → C, then A → C. LSAT arguments often link rules this way. When a conclusion seems to jump, look for the missing link. That is what sufficient assumption questions test.
Where Conditional Logic Shows Up on the LSAT
- Must be true questions: combine rules to find what is guaranteed.
- Sufficient assumption: find the rule that makes the conclusion follow.
- Necessary assumption: find what the argument requires.
- Flaw: spot mistaken reversals and negations.
- Parallel reasoning: match the logical form.
Practice: Translate These
- “No student who skips class passes.” Answer: Skips → not Pass.
- “You will get the job only if you interview well.” Answer: Job → Interview well.
- “The store closes early unless the manager is in.” Answer: not Manager → Closes early.
- “Only seniors can take the course.” Answer: Course → Senior.
FAQ
Is conditional logic still on the LSAT without Logic Games?
Yes. It appears throughout Logical Reasoning, which is two of the three scored sections.
What does “A unless B” mean on the LSAT?
It means if not B, then A. Equivalently, if not A, then B.
What is the difference between “if” and “only if”?
“If” introduces the sufficient condition. “Only if” introduces the necessary condition. “A if B” is B → A. “A only if B” is A → B.
Do I need to diagram on the LSAT?
No, but quick arrows on scratch paper help many students avoid mistakes on harder questions.
Want to see how much these skills could raise your score? Try our free LSAT score calculator and score goal calculator.