Duden 117: "There is an extraordinary range of possible ways in which culture can link sexual identity to corporeality, and interpret corporeality as a sign for the difference between man and woman. No morphological element nor any process such as the flow of semen or the monthly bleeding has been seen at all times and everywhere as unique to a specific gender. Only when such elements are seen by the eye whose vision is shaped by its cultural milieu do they become sexual characteristics. Gender is in the eye of the beholder."