'Knowledge is power,' said the philosopher. The maxim is true; but here is a greater truth: 'Knowledge is safety,'--safety amid the physical ills that beset us,--safety amid the moral pitfalls that environ us. Filled with this thought, we write this book. It is the Revelation of Science to Woman. It tells her, in language which aims at nothing but simplicity, the results which the study of her nature, as distinct from that of man, has attained. We may call it her physical biography. It is high time that such a book were written. The most absorbing question of the day is the 'Woman Question.' The social problems of chiefest interest concern her. And nowhere are those problems more zealously studied than in America, which has thrown aside the trammels of tradition, and is training its free muscles with intent to grapple the untried possibilities of social life. Who can guide us in these experiments? What master, speaking as one having authority, can advise us? There is such a guide, such a master. The laws of woman's physical life shape her destiny and reveal her future. Within these laws all things are possible; beyond them, nothing is of avail. Especially should woman herself understand her own nature. How many women are there, with health, beauty, merriment, ay, morality too, all gone, lost for ever, through ignorance of themselves! What spurious delicacy is this which would hide from woman that which beyond all else it behooves her to know? We repudiate it; and in plain, but decorous language,--truth is always decorous,--we purpose to divulge those secrets hidden hitherto under the technical jargon of science. THE DISTINCTION OF THE SEXES. The distinction of the sexes belongs neither to the highest nor to the lowest forms of existence. Animals and vegetables of the humblest character have no sex. So it is with spirits. Revelation implies that beyond this life sexual characteristics cease. On one occasion the Sadducees put this question to Christ: There was a woman who lawfully had seven husbands, one after the other; now, at the resurrection, which of these shall be her husband? or shall they all have her to wife? He replied that hereafter there shall be neither marrying nor giving in marriage, but that all shall be 'as the angels which are in heaven.' Sexuality implies reproduction, and that is something we do not associate with spiritual life. It further implies imperfection, which is equally far from our hopes of happiness beyond the grave. The polyp, which reproduces by a division of itself, is in one sense more complete than we are. The man is in some respects inferior to the woman; the woman in others is subordinate to man. A happy marriage, a perfect union, they twain one flesh, is the type of the independent, completed being. Without the other, either is defective. 'Marriage,' said Napoléon, 'is strictly indispensable to happiness.'