Plato was already aware of this relationship when he called thought a sublimated sexual impulse, and Buffon likewise when he described love as “le premier essor de la sensibilité, qui se porte ensuite à d’autres objets.” In more recent times, Dr. Santlus, in his valuable essay, “On the Psychology of the Human Impulses” (_Archiv für Psychiatrie_, 1864, vol. vi., pp. 244 and 262), alluded to this combination of the sexual sphere with the highest spiritual interests of mankind under the name of the “function-impulse.” From these intimate relations between sexual and spiritual productivity is to be explained the remarkable fact that certain spiritual creations may take the place of the purely physical sexual impulse; that there are psychical =sexual equivalents= into which the potential energy of the sexual impulse may be transformed. Here belong numerous emotions, such as ferocity, anger, pain, and the productive spiritual activities which find their vent in poetry, art, and religion--in short, the whole =imaginative life= of mankind in the widest sense is able, when the natural activity of the sexual impulse is inhibited, to find such sexual equivalents, the importance of which in the evolutionary history of human love we shall have later to study in further detail. Interesting observations regarding this intimate connexion between the spiritual and the physical procreative impulse are to be found in the work of a thinker who made no secret of his intense sensuality, and in whose life and thought sexuality played a peculiar part--in the work of Schopenhauer. In his “New Paralipomena” he lays stress on the similarity between the work of productive genius and the modification of the sexual impulse peculiar to the human race. In another place in which, as Frauenstädt also insists, he is speaking from personal experience, he writes: “In the days and hours when the =voluptuous= impulse is most powerful, not a dull desire, arising from emptiness and dullness of the consciousness, but a burning longing, a violent ardour, =precisely then also are the highest powers of the spirit available, the finest consciousness is prepared for its intensest activity=, although at the moment when the consciousness has given itself up to desire they are =latent=; but it needs merely a powerful effort to turn their direction, and instead of that tormenting, despairing lust (the kingdom of darkness), the activity of the highest spiritual powers fills the consciousness (the kingdom of light).” Georg Hirth, who, in the section of his “Ways to Love” entitled “Stark-naked Thoughts,” gives in aphorisms an interesting account of the psychology of love, affirms the “delightful phenomenon of a peculiarly active enhancement of our impulse to thought and production,” =after= erotic satisfaction, =after= a fortunate love-night. Very ably, also, has Mantegazza described the spiritual activity produced by a happy and victorious love.[35]