Arguments from Ignorance explores the situations in which the argument from ignorance (also known as the lack-of-knowledge inference, negative evidence, or default reasoning) functions as a respectable form of reasoning and those in which it is indeed fallacious. Douglas Walton draws on everyday conversations on all kinds of practical matters in which the argumentum and ignorantiam is used quite appropriately to infer conclusions. He also discusses the inappropriate use of this kind of argument, referring to various major case studies, including the Salem witchcraft trials, the McCarthy hearings, and the Alger Hiss case. This book makes an original contribution in the areas of argumentation theory and informal logic, contending that, despite its traditional classification as a fallacy, the argument from ignorance is a genuine, very common, and legitimate type of argumentation with an identifiable structure. But the book is also interdisciplinary in scope, explaining many widely interesting and controversial,subjects in artificial intelligence, medical education, philosophy of science, and philosophy of law in a clear way that makes it accessible to a broad range of readers.

A man is sitting inside a warehouse that has a tin roof and no windows. Tin roofs are notorious for making lots of noise inside a building when it rains outside. The man in the warehouse cannot see outside, so he could not tell directly if it were raining at a given time. But he could infer indirectly, using, for example, the following argument: if it were raining now I would know it (by the noise); but I do not know it; therefore, it is not raining now. This type of argument is called an argumentum ad ignorantiam in the logic textbooks, usually translated as the argument from ignorance, or as the appeal to ignorance. Sometimes it is also called lack-of-knowledge inference, negative evidence, or negative proof.

The man's argument from ignorance in the warehouse case seems reasonable enough, but there are some grounds for reservation. First, it is based on ignorance, or so-called negative evidence, in the sense that the premise is he does not hear noise that would normally be the indication that it is raining outside. It is the absence of noise that is the basis of the argument. Second, it is an indirect argument. He does not see the rain or absence of rain outside directly. He can only infer it by presumption from what he hears, or fails to hear, inside. In drawing this conclusion he could perhaps be mistaken. For example, it could be raining softly outside, so that he does . . .