In logic or rhetoric, a fallacy of presumption is any argument that is based on at least one assumption that is faulty or unprovable in the context of the argument. It is different from other types of logical fallacies, such as fallacies of relevance, which may contain true assumptions but draw a faulty conclusion from them. There are any number of different types of arguments that fall into the category of fallacy of presumption, but some of the more common ones include false dichotomies, complex questions, arguments from ignorance, and circular reasoning.
A false dichotomy, also called a false dilemma or a bifurcation fallacy, consists of putting forth only two choices when actually at least one other option is logically possible. The fallacy of presumption comes from denying that other possibilities may exist. For example, an intellectually irresponsible politician might claim, “If you do not vote in favor of tax reform, you do not care about serving the best interests your fellow citizens.” In actuality, of course, someone might choose to vote against a particular tax reform because he or she believes that a different measure would serve the best interests of his or her fellow citizens.
Similarly, a complex question is a question that contains at least one false or unprovable assumption. For instance, the question, “Have you repented of eating too much cheesecake yet?” contains a number of assumptions not necessarily provable by the context. One of these is that the addressee did, in fact, eat too much cheesecake. Also, the “yet” implies that the addressee does intend to repent, regardless of the fact if he or she has already done so. This, in turn, assumes that eating too much cheesecake is an offense of which should be repented.
Circular reasoning comprises yet another form of the fallacy of presumption. In its simplest form, circular reasoning states or implies that something is true because it is true. In real life, of course, most circular arguments are more complex than that. A pacifist, for instance, might claim that a soldier is clearly a murderer because he or she kills people. The underlying assumption is that all killing is murder and that, therefore, someone who kills is a murderer. This first assumption, however, might be questioned by a just-war theorist, thereby making the argument circular.