Page 1 Logical Reasoning Study Guide for the LSAT®
How to Prepare for the Logical Reasoning Section of the LSAT
The Logical Reasoning section of the Law School Admission Test (LSAT) is actually two sections and therefore makes up half of your possible points. You will be presented with short passages to read and a question or two to answer about that passage. This section tests your critical thinking skills—your ability to take apart an argument and see it for what it is. For example, you may be asked to find the flaw in the author’s reasoning, or determine what fact or set of facts would weaken the author’s argument. You will be tested on at least the following additional types of questions: strengthen/weaken an argument/conclusion; method of argument/reasoning; principle questions; inference; points at issue; must be true; parallel reasoning; main point; and paradox.
When answering these questions, be careful to refrain from inserting any of your own prior knowledge about a subject. Also, be sure that your choice answers the question that is posed and is not simply a true statement. Some questions may have several components and you need to find the answer that satisfies all of them.
Terms to Know
Become familiar with these terms and what they mean. Also, be able to utilize them appropriately when answering questions.
An argument is a set of statements that are intended to support and/or prove a conclusion. It consists of one or more premises and a single conclusion. For example:
All mangoes are fruits.
No fruits are square.
Therefore, no mangoes are square.
This is an example of a formal logic argument, or syllogism. It is in the form: All A are B, No B are C. Therefore, no A are C. These are deductive arguments, which are always valid. When you are studying, make sure you pay attention not only to the parts of the argument, but also to their forms.
The premises of an argument are statements that assume the conclusion is true. They are meant to prove the conclusion directly or by inference. For example:
If Christina Aguilera lost her voice, the concert would be ruined.
Christina Aguilera lost her voice.
Therefore, the concert was ruined.
This is loosely in the form: All A are F. X is F. Therefore X is A. This is a fallacy because even if both premises are true, the conclusion can still be false. Right? What if she had a smash warm-up act?
An assumption is a statement for which no proof is offered. It is just intended to be taken as true. It is the bridge between the premise and the conclusion. In other words, arguments rely on assumptions. No proof is offered for the premises set forth themselves to prove the conclusion. They are assumed to be true. For example, here is a classic syllogism:
All men are mortal.;
Socrates is a man.;
Therefore, Socrates is mortal.
Here, the first two statements, the premises, are assumed to be true by the author. So, as you can see, every argument is going to include assumptions.
The conclusion is the statement in the argument, the truth of which is intended to be proven by the argument. Using our earlier syllogism, developed by Aristotle:
All men are mortal.
Socrates is a man.
Therefore, Socrates is mortal.
This is the conclusion. It is what the author is trying to prove or convince others to believe. Based on the premises and inferences that could be drawn from them, Aristotle intended to show that Socrates was mortal. Each argument contains one conclusion. Often, they begin with words such as therefore, wherefore, in conclusion, etc.
Specific Skills to Practice
The Logical Reasoning section of the LSAT is a test of your critical thinking ability. Practice using these skills.
Identifying Parts of an Argument and How They Relate to One Another
As we’ve discussed, there are several parts to any argument: the premise(s), assumption, and conclusion. We have not yet discussed inference. An inference is where a new belief or understanding stems from the first belief or proposition. Inference also draws us from proposition or premise to conclusion. Inferring is drawing conclusions from premises. There are deductive and inductive inferences, the former nearly guaranteeing the validity of its conclusion, while the latter saying merely that it is probable. Understand that assumption is different from inference. Assumptions are statements for which no proof is offered, while inferences are new beliefs formed from premises.
Assumption and inference are similar in that they can lead to the conclusion, but are different in their operation. Both may well be necessary in an argument to reach a logical conclusion. For example:
A mango is a fruit.
John likes fruit.
Therefore, John likes mangoes.
Without getting into the validity of the argument, see that the first premise is also an assumption, as it is taken as true that a mango is a fruit. The same with the second premise. From those premises we infer that John likes mangoes. We form a new belief. This is different from assumption, although as you can see, both are necessary in the argument.
Identifying Similarities and Differences between Reasoning Patterns
On the LSAT, you will be asked to choose the answer that most conforms to the argument pattern of the passage or “stimulus.” It’s called parallel reasoning. This is where having even a rudimentary understanding of argument forms can help a lot. These questions can take more time than most to answer because of the extra text and the time it takes to do the comparisons. Diagramming in the margins can really speed things up and help you keep the questions clear in your head. For example:
Argument: All birds have wings and all penguins have wings, therefore they are alike. Choose:
(a) Birds and penguins are alike; however, they have a lot of dissimilarities.
(b) All birds have wings. Penguins are like birds, therefore they must have wings.
(c) Fish and shrimp must be alike because every fish has eyes and so does every shrimp.
(d) All rats have tails and all monkeys have tails.
(e) Some snakes are cold-blooded and all reptiles are cold-blooded, therefore, snakes are reptiles.
The secret to selecting the right answer is to forget about all of the descriptive words, birds, snakes, etc. and only consider how the original argument is structured. Then find an answer with the same structure. Diagramming the structure of the argument could look like this:
All of A have Q.
All of B have Q.
Therefore, A and B are alike.
Which answer choice fits this pattern?
It is (c).
Every fish (A) has eyes (Q).
Every shrimp (B) has eyes (Q).
Therefore, A and B must be alike.
The order of the argument has been changed, but you can see when you diagram it that it has the same elements.
Developing Well-Supported Conclusions
A well-supported conclusion will have premises that are clearly true, or at least easily accepted as true, and the conclusion will clearly follow, or be easily inferred, from the premises. If your premises are weak or not easily accepted, your argument will likely fail. By the same token, if the conclusion cannot be inferred from the premises, that conclusion will also likely fail. Even so, some might still believe the conclusion, if it is true, but you will have greatly lessened your chances of convincing others of the validity of your argument. For example:
Mangoes are fruit.
All fruits rot.
Therefore, mangoes rot.
The premises are easy to accept and the conclusion follows logically from them. Consider:
Mangoes are fruit.
All fruits taste good.
Therefore, mangoes taste good.
This argument has less teeth. The first premise can easily be accepted as true, but the second may be taken as false by others. The conclusion then may also be taken as false.