Thomas Aquinas, Compendium of Theology

These considerations also refute the error of Apollinaris, who at first followed Arius in refusing to admit any soul in Christ other than the Word of God. However, he did not follow Arius in teaching that the Son of God was a creature; for many things are narrated of Christ which cannot be ascribed to the body, and which are inadmissible in the Creator, such as sadness, fear, and the like. He was, then, at length driven to acknowledge the existence in Christ of some soul which gave sense life to the body and could be the subject of such passions. Yet this soul was without reason and intellect, and the Word Himself took the place of intellect and reason in the man Christ.

This theory is shown to be false on many grounds. In the first place, the very concept of nature is incompatible with the opinion that a non-rational soul is the form of man, whose body nevertheless must have some form. But nothing monstrous or unnatural can be thought of in connection with Christ's incarnation. Secondly, this hypothesis would be inconsistent with the purpose of the Incarnation, namely, the reparation of human nature. Above all, human nature needs to be restored in the intellectual sphere, for that which can have part in sin is precisely the rational soul. Hence it chiefly befitted God's Son to assume man's intellectual nature. Besides, Christ is said to have marveled. But surprise cannot be experienced without a rational soul, and of course is wholly inadmissible in God. Therefore, as the sorrow Christ experienced forces us to admit that He had a sensitive soul, so the wonderment He expressed compels us to acknowledge the existence of a rational soul in Him.