Abstract. The present study applies results and methods of psychobiology to intellectual history. Pascal's syndrome is a depressive neurosis associated with morbid effects of scientific certainty. The syndrome is characterized by self-mortification and conversion experience that represses distressing certainties. The dynamics of the syndrome are assessed from Blake Pascal's psychosis. The ideation of the syndrome is evaluated by reference to the neurology of altered states of consciousness and the biogenic amine hypothesis of depression and mania. The evaluation yields a description of the relation between psychogenic and biogenic factors in the syndrome's etiology.