(3) In the middle of the nineteenth century the conservation of kinetic plus potential energy came to be accepted as a basic principle of physics (Elkana 1974). In itself this does not rule out fundamental mental or vital forces, for there is no reason why such forces should not be “conservative”, operating in such a way as to compensate losses of kinetic energy by gains in potential energy and vice versa. (The term “nervous energy” is a relic of the widespread late nineteenth-century assumption that mental processes store up a species of potential energy that is then released as kinetic energy in action.) However, the conservation of energy does imply that any such special forces must be governed by strict deterministic laws: if mental or vital forces arose spontaneously, then there would be nothing to ensure that they never led to energy increases.