Skipping a great deal, I now take up the third great class of Reasonings, which I call Retroductions. [—] By the third class of reasonings one only infers that a certain state of things may be true and that the indications of its being so are sufficient to warrant further examination. [—] The reason for accepting the Retroductive conclusion, is that man must trust to his power of getting at the truth simply because it is all he has to guide him; and moreover when we look at the instincts of various animals, we are struck with wonder at how they lead those creatures toward rational behaviour. [—]

I do not, at present, feel quite convinced that any logical form can be assigned that will cover all “Retroductions”. For what I mean by a Retroduction is simply a conjecture which arises in the mind.