Update 2015: I have a new site now. On LSAT Hacks I’ve written thousands of free explanations for LSAT preptests. Check it out once you’ve gone through this glossary: http://lsathacks.com

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The LSAT is not a vocabulary test. But there are a few words and concepts that are extremely important to know.

If there’s another word you’re unsure about and would like me to add, let me know.

 

AND/OR: Used in conditional statements. AND requires both conditions to be true. OR requires at least one of the conditions to be true, but both could be true.

e.g. “If you’re rich and in good health, then you’re happy”      R and GH –> H

We don’t know anything about people who are rich, or people who in good health. We only know about people who have both.

“If you’re a parent, you have a boy or a girl”      P –> B or G

A parent always has at least one child, and so they have a boy or a girl. But a parent could also have a boy and a girl, the OR statement doesn’t say you can’t have both.

Conditional Statement: Also known as a “Sufficient-Necessary” or “If-then” statement. The core of LSAT logic. Conditional statements have necessary and sufficient conditions, and you can combine them to form logical deductions.

For example: “All apples are fruit”If you see any apple, then you know it is a fruit. You don’t have to check. It has to be true, because “All apples are fruit”.

This is assuming my statement is true. But on the LSAT, you should always assume conditional statements are true.

Conditional statements will always deal with absolute categories, like “all” or “none”. No middle ground. Otherwise you’re dealing with a “quantifier”.

You can draw condition statements like this: A –> F

This is useful for combining them. Suppose I said: “every fruit is delicious” or “F –> D”

If the necessary and sufficient condition of two conditional statements match, you can combine them, like this: A –> F –> D

“Apples are delicious.” Now go eat one!

Contrapositive: The one correct logical deduction you can make with a single conditional statement. You reverse the terms, and negate them.

“All lions are dangerous”       L –> D

First, reverse the terms:    D –> L

Then put a line through them to negate them:    D –> L

This translates as “If you’re not dangerous, you’re not a lion”.

If there is already a line, remove it. E.g.

“If you don’t study, you will fail”    S –> F

The contrapositive is      F –> S

If there is an AND, change it to OR. If there is an OR, change it to AND.

e.g. “If you’re human, you’re male or female”    H –> M or F

M and F –> H

Contrapositive can be confusing to think about, but they are quite mechanical. Reverse, negate, change AND/OR. Use some real world examples to think through the logic.

Few/A Few: This is like “some”, except only for small numbers. A few refers to 3-5, while few means a small percentage.

The technical details rarely matter. Generally, treat these words like “some”.

Formal Logic: A word used by people who have studied with Powerscore. Anyone who uses this word frequently tends not to understand formal logic. I say this based on tutoring experience.

More formally, “formal logic” refers to the rules that govern all the other terms on this page.

I prefer saying “logic”.

Kaplan/Princeton Review: Jacks of all trades, and masters of none. They teach every test, including regional police exams. Their books can be safely ignored.

Logical Deduction: A conclusion that you get from combining two conditional statements, a conditional statement and a quantifier, or two most statements.

Easiest to explain with examples.

“Some cats are black”      C some B   or  B some C
“All cats have tails”     C –> T

B some C –> T

You can conclude that “some black things have tails” B some T  or T some B

“All battleships are massive”         B –> M
“Anything massive weighs a lot”     M –> L

Join the statement that is the same:      B –> M –> L

“Battleships weigh a lot”. Note that it’s best to stick to one letter for your diagrams.

Logical Opposite: The smallest change you can make to a statement to make it not true. “All” becomes “less than all”, which could be 0-99.9%. The logical opposite of “hot” is “not hot”, rather than cold.

Logical opposites are useful for negation statements.

Logical opposites should not be confused with polar opposites.

LSAC: The authors of the LSAT. Generally quite helpful. Give them a call if you have a question about the test.

The publishers of the “Next ten series of LSAT books”. By far the most useful set of LSAT practice materials.

Available on amazon for ~$20.

                             

LSATs 19-28                          LSATs 29-38                         LSATs 52-61

Many: A quantifier. Like “some”, except the minimum quantity is higher. So while “some” is “one to all” many is more like “15% to all” or “15,000 to all”. “Many” will depend on context. Five students out of a class of twenty counts as “many” students, while five Americans out of 300 million are not many.

This is all theoretical. The LSAT never asks you any precise numerical questions involving many.

The logical opposite is “not many”.

You could make a case that the logical opposite is “few”, but few implies at least “some”, while “not many” could include zero.

Most: More than half. Can include “all”.

The logical opposite of “most” is “not most”, which means “half or fewer”.

Necessary Condition:

Negating a Statement: The smallest change you can make to cause a statement to be false. Use the logical opposite of any logical terms. Mainly this involves quantifiers. A statement of certainty (“I will”) becomes uncertainty (“I might not”).

On necessary assumption questions, the negation of the right answer will always wreck the argument.

Examples:

“My team will win every game.” “No, your team will lose some games”

“The factory will be profitable.” “The factory might not be profitable” or “The factory might lose money”.

Negations can use different words and mean the same thing. You could have said
“Your team will not win every game”, but that is less clear.

Polar Opposites: The complete opposite of a term. Commonly used in real life, but largely useless on the LSAT. The polar opposite of “none” is “all” and the polar opposite of “hot” is “cold”.

Think of the north and south poles, on the opposite ends of the earth. Polar means completely separate.

The LSAT deals in subtleties. Train yourself not to think in polar opposites. There are lands in between the two poles.

Powerscore: Creators of the LSAT “Bibles”. The most complete guides to the LSAT, and far better than Kaplan/Princeton Review/etc.

They also teach a few bad habits. Many students read the Bibles and then try to diagram *everything*. Diagramming is not useful for most logical reasoning questions, especially on the LSAT.

Some students want to study the whole set before trying questions. This is counter-productive. Practical experience is more useful than theory.

Turn to the bibles once you’ve identified weak points. But try the questions first.

Some: A quantifier. It refers to any amount or percentage from 1 to all. Imagine you have eaten one apple out of a whole bushel. You’re asked whether the apples are good. You can say “well, some of them are”, referring to the one you’ve eaten.

So you know at least one is good. It’s also possible that they’re all good. So “some” covers the possibility of 1 to all.

The logical opposite of “some” is “none”.

You can draw “some” statements like this:

“Some apples are green”      A some G     or     G some A

“Some” statements are reversible. However, you rarely need to draw them.

Sufficient Condition: A sufficient condition tells you that something else is true. Knowing that the condition is true it “sufficient” for you to know something else.

“Polar bears are white”

If someone says “that bear is white”, then you don’t know anything else about it. It could be an albino brown bear.

If someone says “that bear is a polar bear”, then you know it is also white. “Polar bear” is the sufficient condition.

Sufficient conditions are indicated by words such as “if, all, each, every, etc.” Also, simply stating a fact about something can be a sufficient condition, as in the polar bear example, or the sentence “cars have wheels.”

The order of the phrase doesn’t matter. Watch for the indicator words. e.g.

“You are a student if you go to school”      School –> student

Student –> school is incorrect. There may be students who don’t go to school, such as home-schooled children.