The influence of Adolphe Quételet and Joseph Guislain, born respectively in 1796 and 1797 in Gent, on the most recent developments of Psychiatry has a paradoxical character. Quételet was a mathematician and astronomer, but his interest in statistics led him to organize the standardized collection on data on an international basis and to initiate the measurement of social and pathological behaviours, two trends which dominate present day epidemiological, nosological and clinical research in Psychiatry. Although Guislain is rightly honoured as the organizer of a human system of treatment for the mentally ill in Belgium, his theoretical ideas about the nature of mental disorders, he considered as reactions to a basic disturbance, whose aspects were determined by the physical and psychological peculiarities of the individual, have been practically forgotten. But they provide an alternative to the now prevalent tendency towards a strictly categorical nosology, whose shortcomings are more and more apparent in empirical biological and clinico-epidemiological research.