[i] The LSAT requires no formal training in logic. This is usually an advantage, but creates some challenges when it comes to identifying logic errors and informal fallacies. A person who is familiar with the traditional names for fallacies may instantly identify a particular flaw as a case of “denying the antecedent” yet still be bewildered by the answer choices that are presented. These flashcards matches examples of actual LSAT answer choices (carefully paraphrased, out of respect for copyright) to the Wikipedia names for the respective fallacy.

[q multiple_choice=”true”] fails to consider that there may be other necessary conditions for the result beyond the specific condition that was identified

[c*] Affirming the Consequent

[f] Correct!

[c] May/Must Fallacy

[f] Not the best choice–but it has been credited as a correct answer for this situation.

[/qwiz]

## Formal and Informal Fallacies

“Flaw” questions involve one of three types of errors–faulty assumptions, logical errors (technically known as “formal fallacies”), and other types of faulty reasoning (the “informal fallacies”). Unscrupulous people have exploited these fallacies for so many centuries that most of them have their own Latin names and all of them have their own Wikepedia page. The following is a list of fallacies that have appeared on LSAT preptests, in alphabetical order by Wikipedia entry.

##### Formal Fallacies

Most logicians use the term “antecedent” for the first term in a conditional statement and “consequent” for the last term. The LSAT never uses these terms, however. All LSAT materials call the “antecedent” the “sufficient term” and the “consequent” the “necessary term.”

##### Informal Fallacies
• False dilemma
• May/Must fallacy (surprisingly, Wikipedia has no entry for this)

## Logical Error Flashcards

[qwiz] [i]

Logical errors (more precisely known as “formal fallacies”) involve errors that can be reduced to symbolic logic, regardless of the actual content of the argument.  This quiz identifies all the formal fallacies found in 10 Actual Official LSAT Preptests Volume V, using Wikipedia’s name for each error.

PowerScore has trademarked their names for these errors, so the first few entries take PowerScore’s definitions and match them to the Wikipedia names.

[q multiple_choice=”true”] Take a conditional statement (A->B) and then negate both terms (/A=>/B). This logical error is known as:

[f] Well, yes, but that doesn’t help us much here, does it? You can find what PowerScore calls this in Chapter 6 of the Logical Reasoning Bible.

[c] Denying the Consequent

[f] No, that one isn’t a fallacy. If the “consequent” (which the LSAT refers to as the “necessary term” is false, the “antecedent” (a/k/a the “sufficient term”) must also be false.

[c*] Denying the Antecedent

[f] Good! The “antecedent” is the first term in a conditional, which the LSAT routinely refers to as the “sufficient” term.

[q multiple_choice=”true”] Take a conditional statement (A->B), assume the second term is true, and conclude that the first term must be true as well.

[f] Well, yes, but that doesn’t help us much here, does it? You can find what PowerScore calls this in Chapter 6 of the Logical Reasoning Bible.

[c*] Affirming the Consequent

[f] Good! The “consequent” is the second term in a conditional, which the LSAT routinely refers to as the “necessary” term.

[c] Denying the Consequent

[f] No, that one isn’t a fallacy. If the “consequent” (which the LSAT refers to as the “necessary term” is false, the “antecedent” (a/k/a the “sufficient term”) must also be false.

[q multiple_choice=”true”] Only people in group A were cured. Therefore if anyone wasn’t cured, he must not have been in group A.

[c*]  Denying the Antecedent

[f] Correct! You can’t get from C->A to /C->/A. That would be like going from “If I live in California then I live in America” to “If I don’t live in California I don’t live in America.”

[c] Appeal to probability

[f] No, that would mean mistaking a condition that MIGHT happen for  a condition that MUST happen.

[/qwiz]

## Flaw Questions

“Flaw” type questions come in three basic forms–assumptions, logical errors, and fallacies. Doing well on flaw questions means doing well on each of these three very different challenges.

##### Assumptions

Assumptions are easiest for people with no special training in logic. It is easy to spot an assumption type answer, since  it tends to start with a phrase like “fails to consider that” or “takes for granted that.” In a typical assumption scenario, the answer choice will provide a new fact that would make a big difference to the argument. You don’t need to be a logic whiz to figure out how that new fact might change things. Since each assumption-type answer is unique to the facts in that particular argument, there is no easy way to train up to do better on assumption answers.

##### Logical Errors

Logical errors, by contrast, do not involve new facts. They are technically known as “formal fallacies,” which means they are wrong because of the “form” of the argument. Any “formal fallacy” can be reduced to symbolic logic so that the actual terms under discussion no longer matter. A stimulus that says “Albion is in Britain” can be rewritten as “A->B.” In a “formal fallacy,” it doesn’t matter whether “B” stands for “Britain” or “Botswana.” For example, if I say “I am in Britain, therefore I am in Albion,” I have committed the logical error that PowerScore refers to as a “mistaken reversal.” To do well on this type of flaw question, you need to do well on conditional logic as a whole. I am working on a flashcard deck for logical errors.

• Logical error flashcards
##### Fallacies

The third type of flaw is a specialized version of logical errors that cannot be reduced to symbolic logic. These “informal fallacies” involve a host of tricks and traps for the unwary. Unscrupulous people have been using these fallacies to dupe people for so many centuries that most of them have Latin names. These Latin names never appear on the LSAT, which adds an unintended degree of difficulty to the test. The LSAT answer choices that describe these informal fallacies can be more bewildering than Latin, especially to people who have some familiarity with the traditional names. To address this problem, I am working on two sets of informal fallacy flash cards–one that identifies all the most common and/or recent fallacies by their Wikipedia names, and another which then connects those names to wording that mimics the LSAT answer choices.

• List of fallacies
• Fallacy flashcards

## Informal Fallacy Flashcards

[qwiz] [i]

Informal Fallacies: This quiz identifies all the informal fallacies in 10 Actual Official LSAT Preptests Volume V. Since many fallacies have more than one name, we have chosen the name used by Wikipedia for each fallacy.

[q multiple_choice=”true”] presents a situation in which only limited alternatives are considered, when in fact there is at least one additional option.

[c]  Sampling bias

[f] No, that would involve some kind of survey error.

[c] Begging the question

[f] No, that would mean assuming what you are trying to prove.

[c*] False dilemma

[f] Good!

[q multiple_choice=”true”]  infers that something is true of the whole from the fact that it is true of some (or even every) part of the whole

[c*] Fallacy of Composition

[f] Good!

[c] Argument from ignorance

[f] No, that would mean using a lack of proof as a way of proving something.

[c] May/Must Fallacy

[f] Wikipedia doesn’t know about this one–it is the error of saying something must be true just because it might be true.

[/qwiz]