Results of Kleist's researchs on brain pathology illustrate his conceptions on brain physiopathology and are disclosed in his "Gehirnpathologie", published in 1934. Kleist's ideas on brain dynamics are determined by clinical reality; the neurological researches serve as a guide to the psychopathological ones. Thus, by stages, Kleist constructs the notions of "brain systems" and their role concerned to neurological and psychic functions. Any nervous function is secured by the concourse of two functional sectors to which correspond proper anatomical substracts. Various anatomical structures, for instance, correspond to the complex execution of automatic motility but, in all these structures, there are two groups of centers with functional significance: one for reception and coordination of stimuli and another with innervatory functions (execution). Such a duality of primordial functions is necessary to complementary action for neurological or psychic functions. This is the mechanism for automatic motility, psychomotility, explicit action, language and abstract thinking. Pathology has allowed Kleist to distinguish two functional sectors designed for the elaboration of abstract thought, namely: the posterior cortical zone (19 and 39 Brodmann's fields) to which correspond receptive-coordinating functions, designed to the working out of concepts. The disturbance of these functions brings out a peculiar disorder, consisting in a difficulty or impossibility to elaborate isolated concepts: paralogical disorder. But man does not think isolated concepts, for these are gathered to form judgments by means of predicative relationships. Those "active" functions for gathering and affecting the discursive thought are related to the anterior brain cortex (Brodmann's field 46). A disturbance of these functions, by a mere dynamic derangement or through anatomical disorganization, brings out the alogical disorder. Although these paralogical and alogical disorders are found in many diseases due to brain lesions it is in Schizophrenia that the paralogical disorder rises in importance and relative purity. Paralogical disorders have often been described with the vague and inappropriate concept of "mental dissociation".