The Commens Dictionary

Quote from ‘Letter to J. H. Kehler’

I am unable yet quite to prove that the three kinds of reasoning I mean are the only kinds of sound reasoning; though I can show reason to think that it can be proved, and very strong probable reasons for thinking that there is no fourth kind. I call the three, Deduction, Induction, and Retroduction; though the last only is a word invented by me.

A scientific inquiry must usually, if not always, begin with retroduction. An Induction can hardly be sound or at least is to be suspected usually, unless it has been preceded by a Retroductive reasoning to the same general effect.