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Harder reasoning · Lesson 10 of 14

Conditional reasoning basics

After this lesson, you can identify sufficient and necessary conditions in a statement, build a valid contrapositive, and tell it apart from an invalid reversal or negation.

9 min video, plus notes and 1 practice question below

In this lesson

• Tell a sufficient condition (the trigger, like a key) apart from a necessary condition (the requirement, like a lock), using a real example: if you get a scholarship, you must have applied
• Read the indicator words that flag each type of condition
• Build a valid contrapositive and spot an invalid reversal or negation
• Diagram tightly packed conditional statements as X arrow Y
• Stick with only what the conditional logic guarantees, without adding outside assumptions

What conditional reasoning is

Conditional reasoning connects 2 ideas so that if one happens, the other must follow. Think of it as a chain reaction: if A happens, then B follows. For example, if you get a scholarship, then you must have applied. Getting the scholarship depends on applying, which is why this relationship breaks into 2 pieces: a sufficient condition and a necessary condition.

A sufficient condition is the trigger. It's enough by itself to guarantee the necessary condition, so in the example, getting the scholarship is sufficient because it kicks off the chain. A necessary condition is what must be true if the sufficient condition happens. Applying is necessary here: you're not getting the scholarship without it.

Picture the sufficient condition as a key and the necessary condition as the lock. Without the lock, the key doesn't do anything. Together they form one complete relationship.

The indicator words that flag each condition

LSAT stimuli are full of indicator words that flag which condition you're looking at. For sufficient conditions, watch for if, when, whenever, all, any, each, and every. These words introduce the trigger, the thing that's enough to guarantee the next step.

For necessary conditions, watch for then, only if, must, required, and depends on. These words point to the requirement, the thing that has to be true for the relationship to hold. Memorize this list. It's your set of road signs for reading conditional statements fast.

Contrapositives vs reversals and negations

One of the biggest traps in conditional reasoning is confusing a contrapositive with a reversal or a negation. A contrapositive flips the order of both conditions and negates both of them, and it means the exact same thing as the original statement. Take this example: if you're a successful entrepreneur, then you innovate. The contrapositive is if you don't innovate, then you're not a successful entrepreneur. Both statements say the same thing.

A reversal flips the order without negating anything, and it isn't logically equivalent. Reversing the example gives you if someone innovates, then they must be a successful entrepreneur, which isn't true: innovating doesn't guarantee entrepreneurial success. A negation keeps the original order but negates both conditions, and it's also not equivalent. Negating the example gives you if someone isn't a successful entrepreneur, then they don't innovate, which also isn't true, since they might innovate in other ways.

Test writers exploit reversals and negations constantly, so train yourself to recognize the difference between a valid contrapositive and an invalid flip.

How to practice it

Mastering conditional reasoning takes practice, not just understanding the concepts. Start by memorizing the indicator words for sufficient and necessary conditions. LSAT logical reasoning rewards pattern recognition more than memorization overall, but this is one area where flashcards can genuinely help if you need them.

From there, practice identifying the sufficient condition and the necessary condition in real questions, wherever they show up in the stimulus. When you get one wrong, go back and figure out exactly where it broke down. Did you miss an indicator word? Did you fall for a reversal or a negation? Pinpointing the exact mistake is what lets you avoid repeating it.

When to diagram and how to spot valid inferences

Beyond drilling the words, you'll sometimes want to diagram a conditional relationship: sufficient condition, an arrow, then the necessary condition. The simplest version is X arrow Y, meaning X requires Y, so if you have X, you're guaranteed to have Y too.

Diagramming is most useful in formal logic, where a stimulus packs in multiple conditional statements written tightly together. You'll see this in inference questions like must be true, in sufficient assumption questions, and in parallel reasoning questions. You won't see it nearly as often in strengthen, weaken, or necessary assumption questions.

When you approach a formal logic question, focus on valid inferences. Train yourself to recognize what you can conclude and what you can't, and stick with what the logic guarantees. Don't bring in outside information or real world assumptions, even ones that sound reasonable. Treat every statement in the stimulus as fact, no matter how strange it seems.

The short version

• Keep the indicator words on a sticky note until they're automatic: if, when, and every trigger sufficient; then, only if, and must trigger necessary. Test yourself blind on 10 sentences before you call them memorized.
• Every time you flip a conditional wrong, note whether you reversed it or negated it. Those are 2 different mistakes with 2 different fixes, and most people only make one of them.
• When a stimulus stacks several conditional statements together, sketch each one as X arrow Y on scratch paper, then answer only from what those arrows force, nothing you'd assume from outside the stimulus.

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