Abductive reasoning was coined by the American philosopher Charles Sanders Peirce around 1865.1,2 Up until this point, philosophers had divided logical arguments into two subclasses. First, you have deduction, or “necessary inference,” where a specific conclusion follows from a general rule. For example, given the premises “All men are mortal” and “Socrates is a man,” deductive reasoning leads us to the conclusion that Socrates must be mortal. Deduction results in “necessary” inferences because it is always correct: it’s about taking rules that apply to a wider population and inferring that they also apply to a random sample.2
In contrast, induction is a type of “probable” inference, extrapolating a general rule based on specific examples. For instance, given the premises “Socrates is a man” and “Socrates is mortal,” induction leads us to the generalized conclusion that “All men are mortal.” This process is the inverse of deduction, making an argument about a population based on a smaller sample, which can often lead to incorrect conclusions.
Deduction and induction capture many of the logical arguments that a person can make, but Charles Sanders Peirce noticed that there was a type of argument that didn’t fit into either of these categories. Imagine, for example, that you go out for a walk, and you notice that the grass is wet. As a first premise, you know that when it rains, the grass gets wet; and as a second premise, you know that the grass is currently wet. You might be tempted to argue that, therefore, it must have rained. There’s a strong possibility that this is the case—but it’s also not the only possible explanation for the grass being wet. This could also be because, say, somebody had their sprinkler on earlier. You’ve made an educated guess, but it is not the sole conclusion to be drawn. This “best guess” reasoning is what Peirce called abduction.2