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The fallacy of presumption is an argument based on an incorrect or unprovable assumption. False dichotomies, complex questions, and circular reasoning are common examples. It involves denying other possibilities.
In logic or rhetoric, a fallacy of presumption is any argument based on at least one assumption that is incorrect or unprovable in the context of the argument. It is different from other types of logical errors, such as relevance errors, which can contain true hypotheses but draw an incorrect conclusion from them. There are several types of arguments that fall into the category of the presumption fallacy, but some of the more common ones include false dichotomies, complex questions, ignorance arguments, and circular reasoning.
A false dichotomy, also called a false dilemma or bifurcation fallacy, is proposing only two choices when in reality at least one other option is logically possible. The fallacy of presumption comes from denying that other possibilities exist. For example, an intellectually irresponsible politician might say, “If you’re not voting for tax reform, you’re not interested in serving the best interests of your fellow citizens.” In reality, of course, someone might choose to vote against a particular tax reform because they believe a different measure would serve the best interests of their fellow citizens.
Similarly, a complex question is one that contains at least one false or unprovable assumption. For example, the question “Do you already regret eating too much cheesecake?” contains a series of hypotheses not necessarily demonstrable from the context. One of them is that the recipient has, in fact, eaten too much cheesecake. Furthermore, the “yet” implies that the recipient intends to repent, whether or not he has already done so. This, in turn, assumes that eating too much cheesecake is an offense that should be regretted.
Circular reasoning includes another form of the presumption fallacy. In its simplest form, circular reasoning claims or implies that something is true because it is true. In real life, of course, most circular arguments are more complex than that. For example, a pacifist might argue that a soldier is clearly a murderer because he kills people. The underlying assumption is that every killing is murder and, therefore, whoever kills is a murderer. This first assumption, however, could be questioned by a just war theorist, thus making the argument circular.
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