False dichotomy

The logical fallacy of false dichotomy or false choice occurs when one provides only two (or a few) answers to a question, giving the illusion that these choices exhaust all possibilities, when in fact they don't. Some examples include:


 * How did life arise? It has to be either evolution or creation.
 * In fact, both may be wrong. (Also, don't forget that evolution doesn't explain how life arose!)


 * Based on what he claimed, Jesus was either a liar, a lunatic, or the Lord.
 * This ignores other possibilities, such as "legend" or that he never made such claims that were attributed to him.


 * You're either with us or against us.
 * I might not stand in your way, but cannot actively support your course of action.

It's worth noting that not all logical dichotomies are false. Consider the etymological definitions of Theism and Atheism.


 * Theism = Belief in one or more gods
 * Atheism = "A" + "Theism" = Lacking + (Belief in one or more gods)

As one cannot simultaneously lack belief and believe, this is an example of a valid dichotomy.

"None of the above"
One way of spotting a false dichotomy is to add "none of the above" to the list of possibilities. If it can be eliminated, then the dichotomy is true. For instance:

Either
 * This glass is empty
 * This glass is full
 * None of the above

If a glass is neither full nor empty, it might be half-full, or 18% full, or nearly full, all of which fall under the category of "none of the above". "Empty or full" is a false dichotomy.

Either
 * This glass is empty
 * This glass isn't empty
 * None of the above

"Empty" and "not empty" cover all of the possibilities, so "none of the above" can be eliminated. "Empty or not empty" is a true dichotomy.

Note that it isn't necessary to be able to say what "none of the above" is. For instance, let's say someone says, "Either flying saucers exist, or a lot of intelligent, well-respected people are lying." You may not be able to think of an alternate explanation, but it is not clear that "flying saucers" and "good people lying" cover all the bases. There might be a third explanation that you haven't thought of yet. Thus, "none of the above" cannot be eliminated. Intelligent people might be sincerely mistaken.

As a rule, any time a list of possibilities is presented as "A or B", rather than "A or not-A", be on the lookout for a false dichotomy.