The dualism of the two worlds appears in sharp outlines in the narrower field of the life of man. The soul of man belongs to both worlds. On the one hand, it belongs to the world of Becoming and partakes of that world through its sense-perceptions, desires, and their pleasures. In this lower world it is the principle of life and motion; it is that which moves itself and other things. On the other hand, it shares in the world of Being through its intuitive reason or knowledge. It shares in the instability and change of psychical phenomena; it also possesses the immutability of reality. Through its perceptions it constructs its “opinions” or inferences of changing phenomena; through its reason it has true knowledge of the eternal Ideas. Therefore the soul must bear in itself traits that correspond to the two worlds. Plato conceives man to have an irrational and a rational nature; and he divides the irrational nature into two parts,――the noble irrational part and the ignoble irrational part. The rational part of man is the reason, the noble irrational part is the will, the ignoble irrational part is the sensuous appetites. { Rational nature ＝ reason Man { Irrational nature { Noble ＝ will { { Ignoble ＝ sensuous appetites This is the celebrated doctrine of the “three parts” of the soul. Are they three parts or three functions of the soul? Plato is not clear as to this point. He sometimes speaks of them as three divisions, and treats them as separable in such a way that only the reason is immortal and the other two parts are mortal. Again, he speaks of the soul as a unity, which carries with it in the next life all three functions. In this latter meaning the three parts are three natures or three different degrees of worth of the unitary soul.