ULRIKE HAHN and MIKE OAKSFORD A BAYESIAN APPROACH TO INFORMAL ARGUMENT FALLACIES ABSTRACT. We examine in detail three classic reasoning fallacies, that is, sup- posedly “incorrect” forms of argument. These are the so-called argumentam ad ignorantiam, the circular argument or petitio principii, and the slippery slope argu- ment. In each case, the argument type is shown to match structurally arguments which are widely accepted. This suggests that it is not the form of the argu- ments as such that is problematic but rather something about the content of those examples with which they are typically justified. This leads to a Bayesian reanalysis of these classic argument forms and a reformulation of the conditions under which they do or do not constitute legitimate forms of argumentation. “The most remarkable feature of the history of the study of fallacies is its con- tinuity. Despite the waves of disinterest and rebellion that phase and punctuate it, and despite fundamental changes in logical doctrine, the tradition has been unquenchable. The lesson of this must be that there is something of importance in it.” Hamblin (1970, 190). Lists of so-called fallacies of argumentative discourse date back to Aristotle’s Sophistical Refutations and his Topics, and have received further additions throughout the ages. There is disagree- ment on the appropriate definition of “fallacy” (see e.g., Eemeren and Grootendorst 2004 for discussion), but the kinds of things intended is clear: petitio principii (or ‘question begging’), arguments from author- ity, ad hominem arguments, or Locke’s argumentum ad ignorantium (argument from ignorance). These fallacies have accumulated in logic textbooks and have been dubbed “informal” fallacies because it has not been possible to give “a general or synoptic account of the tra- ditional fallacy material in formal terms” (Hamblin 1970, 191). The fallacies existence in logic textbooks has been guaranteed by the fact that they seem to highlight important considerations outside formal logic. Like the Aristotelian Topics, the fallacies shall be “superannu- ated when we see what their function is and how it can be fulfilled in a modern idiom” (Hamblin 1970, 191). This question has boiled down [241] Synthese (2006) 152: 207–236 Knowledge, Rationality & Action 241–270 © Springer 2006 DOI 10.1007/s11229-005-5233-2