Retroduction is the passage of the mind from something observed or attentively considered to the representation of a state of things that may explain it. Its conclusion is usually regarded as a more or less likely conjecture; but it may be a mere suggestion of a question or, on the other hand, it may be the most confident of convictions. The essential point is that the consideration of what is observed or known produces some representation of something not so known. This kind of reasoning is justified by two propositions taken together. One is that man’s mind which is a natural product formed under the influences which have developed Nature (here understood as including all that is artificial,) has a natural tendency to think as Nature tends to be. This must be so if man is ever to attain any truth not directly given in perception; and that he is to attain some such truth he cannot consistently, nor at all, deny. The other proposition is that no other process of deriving one judgment from another can ever give any substantial addition to his knowledge; so that, if he is to reason at all, we must assume that this kind of reasoning succeeds often enough to make it worth while; since it certainly is not worth while to leave off reasoning altogether.