But with the advent of Christianity the spirit was sharply dissociated from nature. The original dualism of law of the spirit and law of the flesh, of grace and nature, rescued man at the very beginning from the tyranny of merely natural things, and announced a kingdom of the spirit which "is not of this world." And it is not in fact "of this world," if by world we mean what the word ordinarily implies,--the world which confronts us, and which we can point out to ourselves and to others; the world which, being the object of our experience, is the direct antithesis of what we are, subject of experience, free personality, spirit, Christian humanity. Man, in this Christian conception, in this opposition to nature and to the experimental world, overcomes what within his own self still belongs to nature, subdues that part of him which because natural appears as the enemy of freedom and of the finality of the spirit; as the seducer and the source of guilty wiles which clip the wing of man's loftier aspirations and weigh him down into a beast-like subjection to instinct. He therefore tends to underrate physical education, and sacrifices it to the demands of the spirit. He does not completely neglect the question of the behaviour of man towards physical nature; he could not, since his very dualism is possible only on condition that he correlate the two terms of the opposition. But finding that his attempt to attain freedom and realise his spiritual destiny is thwarted by the natural impulses of the senses, in which the life of the body is made manifest, he decides to remove these hindrances and to clear the way which leads to spiritual salvation. He does then take the body into consideration, but simply to check its instincts and control its sensuous appetites. By the discipline of self-mortification, under the guidance of an unbending will, he subdues the flesh, and subjects it to the exigencies of the spirit.