The field of Psychiatry encompasses the study of both nomothetic (i.e. submitted to a lawfulness) and idiographic (i.e. unpredictable, submitted to the free will or decisions of the individuals) phenomena. Clinical-diagnostic investigations function at three different speeds: -Adagio: slow gathering of diversified data, pertaining to: .history of malady (or psychological maladjustment); .personal history of the patient; .opinion of members of the family; .opinion of the referring practitioner; .last but not least: opinion of the nursing staff. -Allegro: selective grouping of the most significant symptoms; the putative syndrome is then compared with classical well-recognized diagnostic entities (inferential diagnosis). -Presto: the immediate primal impression; this type of perception deals mainly with a tentative and almost reflex appraisal of the personality; it is based on disputable data. This type of unconscious guess is actually more emotional than rational. Psychiatry cannot rely upon "hard symptoms" (such as a cardiac murmur), nor on images (if the latter are convincing they actually contribute to a diagnosis of organic brain impairment), or on chemical evidence of an underlying metabolic disorder. Dexamethazone suppression test did not hold its promises. Only the shortening of the latency time of the paradoxical (R.E.M.) sleep might be useful. All this contributes to the use of psychotropic drugs as a diagnostic tool. It is legitimate because what is known of the action of these drugs on various neuro-transmitters has given us some insight. Nevertheless simplistic ideas which flourished some twenty of thirty years ago, have not been entirely validated.