LSAT Logic | Sufficient vs Necessary Conditions

A sufficient condition guarantees a result. A necessary condition has to be true for that result to happen, but on its own it doesn't guarantee anything. On the LSAT, words like if, when, and whenever usually signal a sufficient condition. Words like then, must, and required usually signal a necessary one.

We deal with “if-then” statements all the time in everyday life. “If I have another drink or two, I won’t be able to drive safely.” “If I get a higher LSAT score, I’ll be competitive at better law schools.”

The LSAT Logical Reasoning section requires us to think about these statements a little more formally than we’re used to. (If we want to do well on the LSAT’s Logical Reasoning section, we’ll have to think about these statements more formally.)

This article will explain “sufficient” and “necessary” and describe them with a few examples.

 

The “sufficient” condition is often introduced by words such as “if,” “when,” and “whenever.”

Something that’s sufficient is enough to get the job done. However, without more info, we can’t assume that it’s a requirement (necessary).

It might help to think of the sufficient condition as “activating” the necessary condition.

 

The “necessary” condition is often introduced by words such as “then,” “must,” and “required.”

Something that’s necessary has to happen in order for the “sufficient condition” to occur.

However, without more info, we can’t assume that it’s the only thing that has to happen in order to know that the sufficient condition also happened (or will happen).

 

Here are a few examples of the sufficient and necessary conditions in action:

Limos

“If I take a limo, I’ll get where I want to go.”

A limo would be sufficient to get me wherever I want to go.

However, I don’t need fancy forms of transportation. There are other ways to get to where I need to go – I could walk, run, bike, or take the subway, bus, or a taxi.

(In fact, I’d prefer a helicopter or rocket ship over a limo if I had my pick.)

 

Un-friending on Facebook

“If we break up, I’ll un-friend him/her on Facebook.”

Breaking up might be sufficient to un-friend a significant other, but breaking up isn’t required (necessary) to un-friend him/her.

You might un-friend him/her in the middle of a fight without actually breaking up.

 

Becoming President

“If you have a reasonable chance of becoming President of the United States, then you must be affiliated with one of the two major political parties.”

However, being affiliated with the Democrats or Republicans is not enough to become President. You also need to win your party’s nomination and, eventually, the Electoral College.

Frequently asked questions

What's the difference between sufficient and necessary conditions on the LSAT?

A sufficient condition is enough on its own to trigger a result, but it isn't required. A necessary condition has to be true for the result to happen, but by itself it doesn't guarantee the result. The two work together in conditional logic. Once you have that, the next step is the contrapositive, and the 2 mistakes people make with it.

What words indicate a sufficient condition?

If, when, and whenever usually introduce a sufficient condition on the LSAT. That's the part of the statement that activates the necessary condition. The complete list, with the time wrinkle, is in indicator words for sufficient and necessary conditions.

What words indicate a necessary condition?

Then, must, and required usually introduce a necessary condition. It's what has to happen for the sufficient condition to hold, though it may not be the only thing required.

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