This chapter aims to understand how the concept of heredity became a coherent tool for psychiatric genetics. It analyses the historical transformation of statements describing the cause and transmission of ‘hereditary madness’ into a scientific explanation of biological laws of heredity. Paracelsus’ view of hereditary madness was not typical of early modern discourse on generation. But for all the peculiarity of Renaissance medicine, Paracelsus’ view of hereditary madness resonates with the modern reader. The concept of heredity appeared, almost exclusively in the French medical community, in the first two decades of the nineteenth century. The synthesis of biological laws concerning natural variation and reproduction had strengthened the resolve and imagination of alienists to elaborate their own distinctions of heredity. The chapter considers how the application of Mendelian statistics led to further innovations of the statistical style by subjecting genealogical thinking to the laws of Mendelian inheritance.