Plausible reasoning

Plausible reasoning is a method of deriving new conclusions from given known premises, a method different from the classical syllogistic argumentation methods of Aristotelian two-valued logic. The syllogistic style of argumentation is illustrated by the oft-quoted argument "All men are mortal, Socrates is a man, and therefore, Socrates is mortal." In contrast, consider the statement "if it is raining then it is cloudy." The only logical inference that one can draw from this is that "if it is not cloudy then it is not raining." But ordinary people in their everyday lives would conclude that "if it is not raining then being cloudy is less plausible," or "if it is cloudy then rain is more plausible." The unstated and unconsciously applied reasoning, arguably incorrect, that made people come to their conclusions is typical of plausible reasoning.

As another example, look at this scenario: "Suppose some dark night a policeman walks down a street, apparently deserted; but suddenly he hears a burglar alarm, looks across the street, and sees a jewellery store with a broken window. Then a person wearing a mask comes crawling out through the broken window, carrying a bag which turns out to be full of expensive jewellery. The policeman immediately concludes that this person is stealing the jewellery." By what reasoning process does the policeman arrive at this conclusion?

Clearly the policeman's conclusion was not a logical deduction from the evidence. There may be a perfectly valid explanation for everything. For example, it might be that this person was the owner of the jewellery store and he was coming home from a fancy dress competition, and he didn't have the key with him. But just as he walked by his store a passing truck threw a stone through the window; and he was only protecting his own property and not stealing the jewellery. Now whatever be the policeman's reasoning process, it has a certain degree of validity. The evidence did not prove that the person was stealing jewellery, but it did make it extremely plausible. This is an example of a kind of reasoning, often referred to as plausible reasoning, in which most people are generally very proficient.

During the fifth century B.C.E., judicial orators in Greek Sicily developed a method for successfully pleading their cases in such instances in which no eyewitnesses or written documents or other such direct evidence could be produced. They began to base their arguments on the internal or external probability or plausibility of their statements. This new way of arguing was commonly labeled with the Greek term eikós, a term that has been variously rendered as similarity, likelihood, probability or plausibility. The success of the argument depends on the oratorical skills of the speaker, arguments by eikós have often been accused of lack of truthfulness. Here is a classical example of argument by plausible reasoning presented by Aristotle in his Rhetoric:

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