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Types of Argumentation
Retroduction

Retroductive arguments are those in which an explanation is suggested to account for an observed fact or set of facts. The explanation is suggested by what I call a "concomitance," i.e. any type of similarity or co-occurrence, including location in time or space, but not restricted to these. For example, "Jones was in the building during each of the murders. Perhaps he is the killer," or "The blood on the victim's shirt matches Jones' blood type. Perhaps Jones is the killer." Because retroductive arguments turn only upon observed concomitances, and concomitances can always be merely coincidental, retroductive arguments run a high risk of being mistaken. Hence, in terms of establishing the truth of the conclusion, retroduction is the weakest type of argumentation. However retroduction is the only type of argumentation that suggests new connections in the structure of the world, so without this type of argumentation the growth of knowledge would be impossible.

 
In retroduction the major premiss is a RULE, as it is in deduction - a general statement about the structure of the world. However, the minor premiss is the RESULT of an observation - a fact, often a surprising fact, that we are attempting to explain. A CASE, subsuming the subject of the result under a known rule, suggests a possible explanation. A retroduction moves from RULE and RESULT to CASE:

 
All men are mortal.
 

 

 A RULE - a known regularity (and it has a concomitance with the observation below about Socrates).
 
Socrates is mortal.

 

 A RESULT of observation. Poor Socrates!

 
Hence, Socrates is a man.

 

 A CASE - the fact that Socrates is an instance of this class could explain his death as a predictable instance of a known regularity.

 

 
In addition to the identification of fallacies, it is possible to critique the comparative strength or weakness of retroductive arguments (as compared to other retroductive arguments). Click here for an explanation of the critique of Retroductive arguments.
I should probably note that the more common term for hypothetical reasoning - among those current logicians who recognize this type of reasoning to be separate from Induction - is Abduction. Peirce himself originally referred to this type of reasoning as "hypothesis." He later coined the term "abduction," and used this term during the 1880s. By 1896 he had abandoned this term in favor of "retroduction," which he used for the remainder of his life. His explanation for the change appears in an unpublished (and undated) manuscript, MS 857, in which he says:
 
  "I have hitherto called this kind of reasonings which issues in explanatory hypotheses and the like, abduction, because I see reason to think that this is what Aristotle intended to denote by the corresponding Greek term '[apagoge]' in the 25th chapter of the 2nd Book of his Analytics […] But since this, after all, is only conjectural, I have on reflexion decided to give this kind of reasoning the name of retroduction to imply that it turns back and leads from the consequent of an admitted consequence, to its antecedent."


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