5.5.3 Fallacies of Unwarranted Assumption

Fallacies of unwarranted assumption occur when an argument relies on a piece of information or belief that requires further justification. The category gets its name from the fact that a person assumes something unwarranted to draw their conclusion. Often the unjustified assumption is only implicit, which can make these types of fallacies difficult to identify.

False Dichotomy

False dichotomy, or “false dilemma,” occurs in an argument when a limited number of possibilities are assumed to be the only available options. In the classic variation, the arguer offers two possibilities, shows that the one cannot be true, and then deduces that the other possibility must be true. Here is the form:

1. Either A or B must be true.
2. A is not true.
3. Therefore, B is true.

The form itself looks like a good argument—a form of disjunctive syllogism. But a false dichotomy is an informal fallacy, and such errors depend upon the content of arguments (their meaning and relation to the world) rather than the form. The problematic assumption occurs in premise 1, where it is assumed that A and B are the only options. Here is a concrete example:

A citizen of the United States either loves their country, or they are a traitor. Since you don’t love your country, you are a traitor.

The above argument assumes that loving the United States or being a traitor are the only two possible options for American citizens. The argument assumes these options are mutually exclusive (you cannot be both) and jointly exhaustive (you must be one or the other). But this position requires justification. For example, a person can have mixed emotions about their country and not be a traitor. False dichotomy is poor reasoning because it artificially limits the available options and then uses this artificial limitation to attempt to prove some conclusion. A false dichotomy may include more than two options. The important thing to remember is a false dichotomy limits options in an argument without justification when there is reason to think there are more options.

Begging the Question

Begging the question occurs when an arguer either assumes the truth of the conclusion they aim to prove in the course of trying to prove it or when an arguer assumes the truth of a contentious claim in their argument. When the former happens, it is sometimes called circular reasoning. Here is an example:

1. The Bible states that God exists.
2. The Bible is true because it is divinely inspired.
3. Therefore, God exists.

The problematic assumption occurs in premise 2. To say the Bible is “divinely inspired” is to say that it is the word of God. But the argument aims to prove that God exists. So premise 2 assumes that God exists in order to prove God exists. This is patently circular reasoning. The name “begging the question” is confusing to some students. One way to think about this fallacy is that the question is whatever is at issue in a debate or argument. Here the question is “Does God exist?” To “beg” the question means to assume you already know the answer. The above argument assumes the answer to the question it is supposed to answer.

The name “begging the question” makes more sense for the second form of the fallacy. When a person begs the question in the second sense, they assume the truth of something controversial while trying to prove their conclusion. Here is an example you might be familiar with:

1. The intentional killing of an innocent person is murder.
2. Abortion is the intentional killing of an innocent person.
3. Therefore, abortion is murder.

This is a valid argument. Structurally, it uses good logic. However, the argument is an example of begging the question because of premise 2. Much of the debate over abortion revolves around the question of whether a fetus is a person. But premise 2 simply assumes that a fetus is a person, so the argument begs the question “Is a fetus a person?”