The growing complexity of life leads to the same end. Where the conditions of life are simple, and the experiences through which people pass are often repeated, and where, moreover, the amount of positive knowledge current is small, conclusions are reached rapidly, and the feeling of confidence in one's own opinions is not checked by seeing others draw different conclusions from the same premises. Under such conditions an opinion once formed is not easily or quickly changed. Experience which makes for wider knowledge makes also for greater caution in forming opinions and a greater readiness to tolerate conclusions of an opposite character at which others may have arrived. Finally, on the purely intellectual side one must reckon with the growth of new ideas, and of knowledge that is in itself quite inconsistent with the established creed. If the primary reason for killing the heretic is that he is a social danger, one who will draw down on the tribe the vengeance of the gods, the strength of that feeling against the heretic must be weakened by every change that lessens men's belief in the power of their deity. And one must assume that every time a fresh piece of definite knowledge was acquired towards the splendid structure that now meets us in the shape of modern science there was accomplished something that involved an ultimate weakening of the belief in the supremacy of the gods. The effect is cumulative, and in time it is bound to make itself felt. Religious opinion after religious opinion finds itself attacked and its power weakened. Things that were thought to be solely due to the action of the gods are found to occur without their being invoked, while invocation does not make the slightest difference to the production of given results. Scientific generalizations in astronomy, in physics, in biology, etc., follow one another, each helping to enforce the lesson that it really does not matter what opinions a man may hold about the gods provided his opinions about the world in which he is living and the forces with which he _must_ deal are sound and solidly based. In a world where opinion is in a healthy state of flux it is impossible for even religion to remain altogether unchanged. So we have first a change in the rigidity of religious conceptions, then a greater readiness to admit the possibility of error, and, finally, the impossibility of preventing the growth and expression of definitely non-religious and anti-religious opinions in a community where all sorts of opinions cannot but arise.