They call it deduction.

I opined that somewhere in this island was a gentleman whom we will call Mr X, and that, pursuing the line of business he did, he must have certain characteristics.

I considered very carefully just what sort of personage he must be.

I had noticed that his device was apparently the Double Bluff.

That is to say, when he had two courses open to him, A and B, he pretended he was going to take B, and so got us guessing that he would try A.Then he took B after all.