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Page 1 Logical Reasoning Study Guide for the LSAT

How to Prepare for the Logical Reasoning Section of the LSAT

General Information

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.

Argument

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.

Premise

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?

Assumption

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.

Conclusion

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.

Using Analogy in Reasoning

When you argue by analogy, you show that at least two things (ideas, things, people) are similar in one or more ways and that because of that they must also be similar in another way. For example:

Mangoes and Papayas are tropical fruits that taste good.
Mangoes are good for you.
Papayas are good for you.

The form of these arguments is generally:

1-4* all have properties A and B.
1-3 have property C also.
Thus, 4 probably also has property C.
*(for illustration purposes)

This is exactly what we’ve done in our example.

Mangoes (1) and Papayas (2) are tropical fruits (A) that taste good (B).
Mangoes (1) are good for you (C).
(Therefore) Papayas are good for you (C).

There are several things to keep in mind when making and analyzing analogical arguments. The more instances of similarity that you have, the stronger your argument. In other words, if we had three or four fruits that tasted good and were good for you instead of just one, that would be better. The similarities must actually be relevant to the conclusion. Also, you want as few dissimilarities as possible because they weaken your conclusion. Lastly, and this is by no means the last factor affecting analogical arguments, diversity of instances of similarity. The more different instances you have with the relevant similarities, the stronger your conclusion. In other words, if five different people all saw the same movie at different times, in different theatres, some in IMAX , 3D, and 2D and all gave it five stars, that is some diversity.

Identifying Areas of Disagreement or Misunderstandings in an Argument

Commonly called “point at issue,” this question will ask you to determine the point of disagreement between two speakers in a passage. The most important thing to remember is that it is unlikely to be their conclusions, but rather premises, inferences, or assumptions. Go through the arguments and find where the disagreement lies, without supplying any additional information yourself, and you will have your answer. For example:

Gregory: Texting while driving is much worse among teenage girls than any male age group. Recent accident data have confirmed this. Cell phones should be banned.
Olivia: Texting while driving is a danger to us all. However, cell phones serve a legitimate purpose otherwise. Cars should be equipped with cell reception blocking technology to prevent the use of cell phones while driving.

So, what do they disagree on? They agree that texting while driving is bad. They agree that something should be done about it. They disagree that cell phones should be banned, or put another way, they disagree that cell phones should be allowed to remain among the populace. Be assured that the questions on the LSAT will not be so simplistic. However, if you stay within the argument and look for the parts that overlap, you should be able to tell the agreements from the disagreements.

Explaining the Effect(s) of Additional Evidence on an Argument

As discussed earlier, you can have deductive or inductive arguments supporting your conclusion. In the case of an inductive argument, where you have not guaranteed your outcome, but merely inferred that it is probable, additional evidence could very well tip the scales in either direction. In fact, one way to determine if an argument is, in fact, inductive is to consider whether additional evidence would make a difference. In the case of a valid deductive argument, if its premises are true, if the conclusion is based on the premises, it must be true. Therefore, additional evidence will not alter the conclusion. You will be asked questions regarding whether certain “evidence” will strengthen or weaken an argument. You might first try to assess whether it is an inductive or valid deductive argument as we have just discussed. Additional evidence directed toward the argument’s assumption(s) can also strengthen or weaken it dramatically.

Identifying Assumptions Used in Presenting an Argument.

In many arguments, the underlying assumption (See: Assumption, above) will not be explicit, or stated, but implicit, or hidden. Learning to spot these assumptions can be the key to finding the issue(s) in an argument. For example:

Abortion is wrong because it is killing.

The implied or implicit assumption is All killing is wrong. The argument is invalid without this assumption. This is not to say that this is the end of the argument, just a statement on form. Finding these assumptions requires first finding the premises and conclusion. You do this because, as we’ve discussed, the assumption is the bridge between them. Some words that signify premises may be because, since, as, in view of, and so forth. Words that signify a conclusion may be accordingly, clearly, hence, so, thus, and so on. When you have identified the premises and conclusion, determine if the argument is valid. (See: Argument above, generally.) If so, the assumption will have been shown explicitly from the premises and the conclusion follows therefrom. If it is not valid, what premises are required to make it so? These are the implicit or hidden assumptions.

Finding and Using Principles and Rules in an Argument

Some would argue a difference between principles and rules in philosophy and law. Principles are norms that can be satisfied based on legal and factual possibility. They are what we use to determine which rule to apply. Rules are norms or standards that either do or do not apply and will or will not be fulfilled. Black and white. However, the rules of logic, and therefore critical thinking, are a bit different. For example:

If A then B
A
Therefore B

This is the form called “modus ponens.” If you find an argument in this form, it will be a valid argument in that its conclusion follows from its assumption(s). So, if the assumption(s) are true, the conclusion must be true. There are several valid forms you should know, including “modus tollens,” “disjunctive syllogism,” “hypothetical syllogism,” “constructive dilemma,” and “destructive dilemma.” Knowing these forms will help you recognize a valid argument and apply your analysis of the truth or falsity of the assumptions to determine the truth of the conclusion. (We will discuss fallacies below.)

Finding and Explaining Flaws in an Argument

Flaws in arguments, or errors in reasoning, are called “logical fallacies.” There are a number of logical fallacies one can fall prey to in an argument. You would do well to know at least the most common of them. For example: “Begging the Question.” This is also known as the “circular argument” because the premises attempt to prove the conclusion with the conclusion itself. In other words, the conclusion is embedded within the premises as a way to reach the same conclusion. For example:

Violent video games don’t hurt children because children are no more violent or agitated after playing them.

Another common fallacy is the “Slippery Slope” where the author begins with a proposition then follows it with a progression of unsupported propositions to an “inevitable” conclusion. For example:

Women shouldn’t eat grapes. If they eat grapes, they will eat M&M’s. Then they will gain forty pounds and their husbands will leave them. Then they will fall behind on the mortgage and lose their homes. Women cannot eat grapes or we will have a greatly increased homeless population.

You see why it is called a slippery slope? There will be times that the offending premise(s) is implied. You will then have to rely on your skills at pulling out the implied premise, then spotting the fallacy. Here is where knowing your valid forms helps, as well.

Finding Explanations in an Argument.

First, it is important to understand that explanations and arguments, though quite similar and easily merged, are not the same. And, it can be difficult to tell them apart. An explanation is made up of two parts: the thing, event, phenomenon to be explained, explanandum, and the part that does the explaining, explanans. One way to spot an explanation is to see if any new knowledge is offered such that one could learn from it, rather than evidence to convince you of the truth of a conclusion. Because argument and explanation are both forms of reasoning, it can be easy to get them confused. For example:

My brother is sick.

If this were going to be an argument, it would go on to offer grounds, reasons, or evidence to answer the question “How do you know?”

Because my brother has a fever of 102, my brother is sick.

This is an argument because it offers evidence to support the conclusion that “my brother is sick.” Sounds like a definition though, doesn’t it? Don’t forget those implied premises.

My brother has a fever of 102.
All persons with fevers of 102 are sick.
Therefore my brother is sick.

If the statement were to be made into a definition, it would offer a cause to answer the question “Why?”

My brother is sick with a fever of 102 caused by his body raising its own temperature to fight off invading bacteria.

In this case, you were presented with new facts so that you would learn from them, not to prove that “my brother is sick.”