This suspicion, this depreciation of human nature was the great error of the mystics. This view was as fatal as the inverse view of pagan antiquity. The model of the perfect life according to Greek philosophy is a life in conformity with nature. To aim at the harmonious development of man was the precept of the ancient Academy, formulated by Plato. The Stoics and the Epicureans had adopted the same principle. Physical nature is considered as good. It gives us the type, the rule, and the measure. The moral rule itself is exactly appropriate to the physical nature. We may say that pagan morality was hygiene, the hygiene of the soul and the body alike; the _mens sana in corpore sano_ gave individual and social direction. The Rationalists, the philosophers of the eighteenth century, such as Baron d’Holbach and later W. Von Humboldt, Darwin, and Herbert Spencer, have adopted analogous views. If these views have been contested, it is because of the imperfections or aberrations of the natural instincts of man. Also, if we wish to base individual family or social morality on the natural instincts of man, it must be specified that these instincts are to be regularized. We must necessarily appeal from the imperfect instincts of the present to the perfected instincts of the future. Their perfection, moreover, will only be a more exact approximation to the real nature of man, and he, having avoided by the aid of science the accidents which cause disease and senile decrepitude, will enjoy a healthy youth and an ideal old age.