Retroduction is one of the three basic types of reasoning. Retroductive arguments are those in which an explanation is proposed to account for an observed fact or group of facts, based upon what I call a "concomitance," i.e. any type of similarity or co-occurrence, including (but not limited to) location in space and time. For example, "Jones was in the building at the time of the murder. Perhaps he is the killer," or "The blood on the victim's shirt matches Jones' blood type. Perhaps Jones is the killer." In the second example, the similarity of blood type is the concomitance on which the inference turns.

The persuasive power of a retroductive argument depends upon the ability of the argument to render striking and unusual events ordinary and predictable, by fitting them into the patterns suggested by concomitances. The force of the argument turns on its ability to make the universe feel regular, patterned, and understandable (even if this feeling is, in some cases, only an illusion).

Because any concomitance, however striking and unusual it may be, could turn out just to be a coincidence, retroductive reasoning not only cannot guarantee that its conclusion is true, given the truth of its premisses, it cannot even give us a plausible estimate of how probable the conclusion is, given the truth of the premisses. All it can do is suggest that the conclusion might be true, given the truth of the premisses. Hence, retroduction is rightly considered the weakest type of reasoning.

On the other hand, unlike both deduction and induction, retroduction suggests new ideas, some of which may, after testing by means of induction, turn out to be true. Hence retroduction is the most useful kind of reasoning in terms of learning more about the world around us. It can not only confirm what we already think, but actually change what we think. Retroduction is the kind of reasoning involved in discovery and invention. We could not get along without it.


The Syllogistic Model:

In a retroduction the minor premiss is an OBSERVATION, usually of a surprising fact, i.e. something that catches our attention and demands an explanation. The major premiss, as it is in deduction, is a RULE, i.e. a general statement, suggested by the observation. The conclusion is a CASE which asserts that the surprising fact might be explained by fitting it into the general pattern provided by the rule. For example...

All men are mortal.            - A RULE about men.
Socrates is mortal.             - An OBSERVATION.  Poor Socrates!
So, Socrates is a man.        - A CASE fitting Socrates into one possible pattern that would explain his death. 


Critique of Retroduction:

Since the goal of a retroductive argument is to make the world feel patterned and regular, the critique of retroductive reasoning turns upon how satisfactorily the argument achieves this result. However, because retroductions are so weak, and never offer their conclusions as more than a suggestion, it is not possible to strengthen or weaken individual retroductive arguments. Rather, what we can do is compare two or more retroductive arguments to judge which is most satisfying. What the arguments have in common is only their minor premiss: the fact(s) to be explained.

Any hypothesis might be considered better than no hypothesis at all, and even a bad guess can at least give an investigation a place to start - which is really all we ask of Retroduction. This raises the interesting question whether there can even be such a thing as a retroductive fallacy. However, like the other forms of reasoning, there are certain types of arguments that are generally recognized as fallacious. A retroductive fallacy occurs when a proposed hypothesis is likely to compare unfavorably to almost any other hypothesis - that it would be accepted only in the near complete absence of alternative suggestions.



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