# Abductive Reasoning

The great logician Charles Sanders Peirce coined the word "abduction" and defined it as a new kind of inference (distinct from deduction and induction).

Let us now consider non-necessary reasoning. This divides itself, according to the different ways in which it may be valid, into three classes: probable deduction; experimental reasoning, which I now call Induction; and processes of thought capable of producing no conclusion more definite than a conjecture, which I now call Abduction. - C. S. Peirce

A more modern view takes into account various pragmatic and contextual clues to estimate the "strength" of an abductive conclusion, that is, an explanation:

Abduction, of inference to the best explanation, is a form of inference that goes from data describing something to a hypothesis that best explains or accounts for the data.

We take abduction to be a distinctive kind of inference that follows this pattern pretty nearly:

D is a collection of data (facts, observations, givens).
H explains D (would, if true, explain D).
No other hypothesis can explain D as well as H does.
... Therefore, H is probably true.

The strength of an abductive conclusion will in general depend on several factors, including:

• how good H is by itself, independently of considering the alternatives,
• how decseively H surpasses the alternatives,
• how thorough the search was for alternative explanations, and
• pragmatic considerations, including
• the costs of being wrong and the benefits of being right,
• how strong the need is to come to a conclusion at all, especially considering the possibility of seeking further evidence before deciding.

- Josephson & Josephson, Abductive Inference

Image: Charles Sanders Peirce, from Wikipedia.

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