In the theory of psycho-analysis we have no hesitation in assuming that the course taken by mental events is automatically regulated by the pleasure principle. We believe, that is to say, that the course of those events is invariably set in motion by an unpleasurable tension, and that it takes a direction such that its final outcome coincides with a lowering of that tension that is, with an avoidance of unpleasure or a production of pleasure.

The Standard Edition of the Complete Psychological Works of Sigmund Freud (ed. 1955)