What is a Fallacy?

Researchers disagree about how to define the very term "fallacy". Focusing just on fallacies in sense (a) above, namely fallacies of argumentation, some researchers define a fallacy as an argument that is deductively invalid or that has very little inductive strength. Because examples of false dilemma, inconsistent premises, and begging the question are valid arguments in this sense, this definition misses some standard fallacies. Other researchers say a fallacy is a mistake in an argument that arises from something other than merely false premises. But the false dilemma fallacy is due to false premises. Still other researchers define a fallacy as an argument that is not good. Good arguments are then defined as those that are deductively valid or inductively strong, and that contain only true, well-established premises, but are not question-begging. A complaint with this definition is that its requirement of truth would improperly lead to calling too much scientific reasoning fallacious; every time a new scientific discovery caused scientists to label a previously well-established claim as false, all the scientists who used that claim as a premise would become fallacious reasoners. This consequence of the definition is acceptable to some researchers but not to others. Because informal reasoning regularly deals with hypothetical reasoning and with premises for which there is great disagreement about whether they are true or false, many researchers would relax the requirement that every premise must be true. One widely accepted definition defines a fallacious argument as one that either is deductively invalid or is inductively very weak or contains an unjustified premise or that ignores relevant evidence that is available and that should be known by the arguer. Finally, yet another theory of fallacy says a fallacy is a failure to provide adequate proof for a belief, the failure being disguised to make the proof look adequate.

Other researchers recommend characterizing a fallacy as a violation of the norms of good reasoning, the rules of critical discussion, dispute resolution, and adequate communication. The difficulty with this approach is that there is so much disagreement about how to characterize these norms.

In addition, all the above definitions are often augmented with some remark to the effect that the fallacies are likely to persuade many reasoners. It is notoriously difficult to be very precise about this vague and subjective notion of being likely to persuade, and some researchers in fallacy theory have therefore recommended dropping the notion in favor of "can be used to persuade."

Some researchers complain that all the above definitions of fallacy are too broad and do not distinguish between mere blunders and actual fallacies, the more serious errors.

Researchers in the field are deeply divided, not only about how to define the term "fallacy" and how to define some of the individual fallacies, but also about whether any general theory of fallacies at all should be pursued if that theory's goal is to provide necessary and sufficient conditions for distinguishing between fallacious and non-fallacious reasoning generally. Analogously, there is doubt in the field of ethics whether researchers should pursue the goal of providing necessary and sufficient conditions for distinguishing moral actions from immoral ones.