This paper intends to examine some opinion of Archer-Hind which are involved in his commantary on ``necessity`` of Plato`s Timaeus. His opinion is as fallows: There is a cardinal doctrine of Plato which means that the only source of mouvement is soul. Therefore there is not any resisting power in matter against uovs (reason). Necessity or errant cause is the forces of matter originated by vovs (reason), the total sum of physical laws which govern the material universe, the laws which govern in the form of plurality. All nature`s forces must follow their proper impulse according to the conditions in which they are for the time being. But within these laws we see that things do not always work for the best. That fact that necessity is persuaded by reason means, according to Archer-Hind, that necessity is a mode of the operation of intelligence. Archer-Hind also maintains that necessity is the errant cause and though nessecity is working stricktly in obedience to a certain law, it is for the most part not inscrutable to us as if it acted from arbitary caprice. The key point of Archer-Hind`s commantary is based on the supposition that the only source of movement is soul. This paper intends to refute this opinion. In Plato`s Timaeus there are two kinds of movement, the one is order-giving activity, the other is passive movement, and this passivity is origin of disorder. Necessity which is the errant cause is based on the passivity of things. For this reason the other opinions of Archer-Hind are easily refuted. The orignal meanig of necessity is contrary to contingence. The errant cause is necessary to the god who makes the world order, because both are connected as causes of the existence of world.