This book, which is only an essay, because its subject matter is so immense, represents, nevertheless, an ambition: one wanted to enlarge the general psychology of love, starting it in the very beginning of male and female activity, and giving man's sexual life its place in the one plan of universal sexuality. Certain moralists have, undeniably, pretended to talk about "love in relation to natural causes," but they were profoundly ignorant of these natural causes: thus Sénancour, whose book, blotted though it be with ideology, remains the boldest work on a subject so essential that nothing can drag it to triviality. If Sénancour had been acquainted with the science of his time, if he had only read Réaumur and Bonnet, Buffon and Lamarck; if he had been able to merge the two ideas, man and animal into one, he, being a man without insurmountable prejudices, might have produced a still readable book. The moment would have been favorable. People were beginning to have some exact knowledge of animals' habits. Bonnet had proved the startling relationships of animal and vegetable reproduction; the essential principle of physiology had been found; the science of life was brief enough to be clear; one might have ventured a theory as to the psychological unity of the animal series.