development, which is shown in the single consciousness during the individual life, and in the total mental development revealed to us by culture and history. The assertion that is occasionally made, based on dogmatic prejudices--namely, that the law of the constancy of matter, that is valid for the forces of nature, must necessarily keep mental life always at the same level in its total value--this assertion is contradicted by the facts of individual and universal development. That does not naturally exclude the possibility of individual interruptions of the course of development, and, because of these, of retrogressive movements arising, in consequence of the above-mentioned conditions, which govern all mental combinations. This combination of creative growth and strict regularity, which marks our mental life, is shown above all in the fact that, especially with the more complicated processes and the more extensive forms of progress of psychical phenomena, the future resultants can never be determined in advance; but that on the other hand it is possible, starting with the given resultants, to achieve, under favourable conditions, an exact deduction into the components. The psychologist, like the psychological historian, is a prophet with his eyes turned towards the past. He ought not only to be able to tell what has happened, but also what necessarily must have happened, according to the position of events. This point of view has in essentials for a long time been held in practice in the historical sciences. It must be of some value that psychology can show the same law of resultants even in the simplest sense-perceptions and affective-processes, where, in consequence of the simplicity of the conditions, very often the retrogressive deduction turns at the same time into a prophecy of events.