“The manifold possibilities for the further development of complex psychology correspond to the various developmental stages it has already passed through. So far as the experimental aspect is concerned, there are still numerous questions which need to be worked out by experimental and statistical methods. I have had to leave many beginnings unfinished because of more pressing tasks that claimed my time and energies. The potentialities of the association experiment are by no means exhausted yet. For instance, the question of the periodic renewal of the emotional tone of complex-stimulators is still unanswered; the problem of familial patterns of association has remained stuck in its beginnings, promising though these were; and so has the investigation of the physiological concomitants of the complex” (CW18, para. 1137).