Physical and moral system of man, 18th century. The human figure represents the theory published in 1775 by French physiologist Pierre Roussel (1742-1802) in the work 'Systeme physique et moral de la femme'. In this book, Roussel linked aspects of male and female anatomy with human moral characteristics, arguing that women were different from men. This was an example of the 18th-century idea of the 'two-sex model'. This illustration is from an 1809 edition of Roussel's work.