If belief consists in an emotional reaction of the entire man on an object, how can we believe at will? We cannot control our emotions.... But gradually our will can lead us to the same results by a very simple method: we need only in cold blood act as if the thing in question were real, and keep acting as if it were real, and it will infallibly end by growing into such a connection with our life that it will become real. It will become so knit with habit and emotion that our interests in it will be those which characterize belief.

Ch. 21 - The Principles of Psychology (1890)