LSAT Prep Most Important Concept: Sufficient & Necessary

We are now a little under two weeks away, and your LSAT prep should be in full gear. Over these past few weeks, we have been reviewing various types of Sufficient & Necessary statements. So how about we have a mixed review of how to diagram different S & N statements? Let’s begin!

(1) “If one advances confidently in the direction of his dreams, then he will meet with a success unexpected in common hours.” –Henry David Thoreau

We know that “if” introduces the sufficient condition: “one advances confidently in the direction of his dreams.” We also know that the other part of the statement will be the necessary condition: “he will meet with a success unexpected in common hours."

Let’s diagram:


not MSUCH ==> not ACDD

(2) “The only people for me are the mad ones…” –Jack Kerouac

Remember that “the only” introduces a sufficient condition: “people for me.” Thus, the necessary condition in this sentence is: “the mad ones…”

You could rewrite this: if a person is for me, then they are a mad one.

Let’s diagram:

PFM ==> M

not M ==> not PFM

(3) “Nobody dances sober, unless they happen to be insane.” –H.P. Lovecraft

The indicator word “unless” introduces the necessary condition: “they happen to be insane.” The remainder of the sentence must first be negated, and then it will constitute the sufficient condition.

So, how do we negate “nobody dances sober”? Well, the negation of “nobody” is at least one person, i.e. somebody. So the sufficient condition is “somebody dances sober.

We could rewrite this: if somebody dances sober, then they happen to be insane.

Let’s diagram:

DS ==> I

not I ==> not DS

(4) “Only light can drive out darkness.” –Martin Luther King Jr.

Don’t forget that “only” is different from “the only.” As opposed to “the only,” the word “only” by itself introduces the necessary condition: “light.” Thus, the sufficient condition in the above quote is: “can drive out darkness.”

We can rewrite this: If it can drive out darkness, then it is light.

Let’s diagram:

DOD ==> L

not L ==> not DOD

See, that wasn’t difficult at all. On the day of the exam you will be faced with a similar toss up of different Sufficient & Necessary statements on not only Logical Reasoning but also Logic Games.

You’ve got this! Happy Studying!

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