If it really were the case, as popular opinion has tried to establish, that the genius were separated from ordinary men by a thick wall through which… - Otto Weininger

Colour-blindness always extends to the complementary colours. Those who are red blind are also green blind; those who are blind to blue have no consciousness of yellow. This law holds good for all mental phenomena; it is a fundamental condition of consciousness.

As the mental endowment of a man varies with the organisation of his accumulated experiences, the better endowed he is, the more readily will he be able to remember his whole past, everything that he has ever thought or heard, seen or done, perceived or felt, the more completely in fact will he be able to reproduce his whole life. Universal remembrance of all its experiences, therefore, is the surest, most general, and most easily proved mark of a genius.

In the case of complex personalities the matter stands thus: one of these can understand other men better than they can understand themselves, because within himself he has not only the character he is grasping, but also its opposite. Duality is necessary for observation and comprehension.