The notion of mental energy as an account of why we are active at all is as old as the ‘active principles’ of Greek philosophy. It took hold on psychology with the instinct theories of Freud and McDougall and subsequently of the ethologists. The concept was adapted by the behaviour theorists and, under the influence of Cannon, by the energetics school. Currently the tradition is carried by arousal theory. The concept is criticized as being of uncertain ontological status, tending towards hypostatic explanation and as being an application of power engineering principles to a communication system. Current understanding of the reticular system offers little support for its proposed role as an energy source. It is argued that the central question of why we are active at all is an unnecessary one.