I. Assumptions


In the previous chapter, we discussed argument structures and the relations among premises and conclusions. On the other hand, this chapter explores assumptions or ‘implicit premises’ that are not explicitly stated but assumed to be true.

II. Flaws


Identifying the ‘flaws’ in an argument involves determining what is wrong in an argument, which mainly involves logical fallacies. This is one of the most difficult types of questions to master, but this is certainly achievable through practice.

Solving ‘flaw’ questions

  • Identify and understand the idea of the conclusion and premises.
  • Identify assumptions because, from them, you can more easily identify flaws in arguments.
  • Keep an eye out for extreme statements since these statements are harder to defend or prove and therefore, are more susceptible to flaws or fallacies.

Read all the answer choices carefully to find the one closest to your understanding of the flaw.

III. Ad Hominem Flaws

Ad Hominem Flaws

Ad hominem flaws are logical fallacies that involve personal attacks intended to discredit the person making a certain argument. Arguments of this kind are fallacious because instead of attacking the argument of the person, the attack is shifted to the person himself.

· Argumentum ad populum

Argumentum ad populum is a kind of logical fallacy wherein the truth of a statement becomes based on how popular it is or how many people believe it.

· Genetic fallacy

This fallacy means dismissing an argument based on its source or where it came from.

· Appeal to authority

The appeal to authority is a fallacy that bases the validity of a statement on whether it came from an authority. In other words, it is taking the word of an authority on a subject as absolute truth.

· No true Scotsman

This fallacy is dismissing the argument of a person by saying that they are not really a member of your group.

IV. Argumentative Flaws

Argumentative Flaws

Previously, we have discussed fallacies or reasoning flaws involving the relation between an argument and its source or the person saying it. In this part, we discuss flaws involving argumentative techniques.

  • Fallacy of faulty analogy

This fallacy is making an analogy between two things that are not similar or irrelevant to the argument.

  • Straw man fallacy

The straw man fallacy involves giving the impression that one is refuting the opponent’s argument, even though it is not really the argument the opponent presented.

  • Slippery slope

The slippery slope fallacy assumes that if one thing happens, another thing would follow even though in reality, the thing that supposedly would follow does not necessarily have to.

  • Begging the question

Also known as circular reasoning, this fallacy ‘begs the question’ by presenting the conclusion as the premise.

  • Fallacy of equivocation

This fallacy occurs when a word or phrase with more than one meaning is used differently throughout an argument.

  • Either/Or thinking

Also known as ‘black or white’ fallacy, this thinking assumes that there are only two possible ways for something. It assumes that something is either black or white and that there are no other options.

  • “All things are equal” fallacy

The ‘all things are equal’ fallacy assumes that background conditions are equal in all situations.

  • Non sequitur

Arguments that are non sequitur claim that a statement follows from another statement, even though they do not necessarily follow from one another. It presents conclusions as following from a premise or premises when in fact, it does not necessarily have to.

  • Special pleading

Special pleading is making an exception for an argument when it is proven to be false. It is saying that if something refutes an argument, that something is an exception to the rule.

  • Part-to-whole

The part-to-whole fallacy incorrectly assumes that the part of something or quality of something automatically applies to the whole.

  • Fallacy of fallacies

This fallacy involves saying that because an argument is fallaciously argued, it is already incorrect. There are cases wherein an argument is completely valid but the person arguing for it commits a fallacy, which does not mean that the argument then becomes invalid.

V. Sufficient Assumptions

Sufficient Assumptions

Sufficient assumption questions involve identifying the sufficient assumption of an argument. Sufficient assumptions guarantee that the conclusion of an argument is true. You can identify sufficient assumption questions by looking for certain language cues such as the following:

  • Enables
  • Ensures
  • Suffices
  • Allows
  • Would make
  • If

VI. Necessary Assumptions

Necessary Assumptions

Necessary assumption questions involve identifying the necessary assumptions of an argument. Whereas sufficient assumptions guarantee the conclusion of an argument, necessary assumptions are required for the conclusion of an argument to be true.

You can more easily identify necessary assumption questions by familiarizing yourself with certain language cues like the following:

  • Requires
  • Depends on
  • Relies on
  • Must
  • Only if
  • The argument assumes that

Now that you’ve wrapped up the Assumptions chapter, we’re going to move on to some trap question types and then review questions.

Beware: Scope Trap

If you’ve found the main point, you must also identify what is in the range of the argument. Scope is related to more than just the general topic being discussed: it is the narrowing of the topic. Is the article about graduate-school admissions, law school admissions, or helping literature students get into the law school program of their choice? Each step represents a narrowing of the scope.

Let’s look at this critical reasoning question to examine scope.

Apartment building owners argue that rent control should be abolished. Although they acknowledge that, in the short term, rents would increase, they argue that the long-term effect would be a reduction in rents. This is because rent increases would lead to greater profitability. Higher profits would lead to increased apartment construction. Increased apartment construction would then lead to a greater supply of residences, and lower prices would result because potential apartment residents would have a better selection. Thus, abolishing rent control would ultimately reduce prices.

Name an assumption made by the owners:
(Hint: this is a difficult question, but you can eliminate four of the five answers as outside the scope of the argument.)

  • Current residents of rent-controlled apartments would be able to find new apartments once their rents increased.
  • The fundamental value of any society is to house its citizens.
  • Only current apartment owners would profit significantly from market deregulation.
  • New apartment construction will generate a great number of jobs.
  • The increase in the number of apartments available would exceed the number of new potential apartment residents.

Which possible answers are outside of the scope? The scope is the argument that deregulation will increase supply and lower prices. “Name an assumption” means find a direct assumption of the supply/demand argument.

(E) is the only choice that isn’t outside the scope of the argument: price of rents. So, we can choose it without even seeing a question stem. The argument revolves around a supply/demand dynamic in housing, so the answer should as well.

(A) is incorrect because current residents aren’t related to the issue in the argument. (B) and (C) go afield into areas that aren’t related, such as societal ethics. The issue of jobs isn’t mentioned either, which rules out (D). (E) is the only choice directly germane to the paragraph.

Next LSAT: Jun 10/Jun 11


If two questions are the same, then both are likely wrong. This video is from the GMAT critical reasoning section, which is very similar to the LSAT logical reasoning question (GMAT students often review official LSAT questions for a challenge).

Review Questions




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Next LSAT: Jun 10/Jun 11