In the east, several theologians tried to explain the relationship between the two natures of Christ. They were not very successful.
The first was Apollinaris, who was on the Alexandrian side of the argument. He said that the second person of the Trinity, the Word, took the place of the rational soul in Jesus. Basically, Apollinaris was saying that Jesus had a human body, but a divine mind. He asserted that the human mind is subject to great change and prone to fantasize about improper things. Christ's mind was the unchangeable, divine Word. Christ's body was a changeable, human body.
This explanation was eventually rejected. The opposition came for the Antiochene School; Jesus must be truly human. This was necessary because Jesus became human that he might save humanity. If he was only partly human, then humanity could only be partly saved. Gregory of Nazianzus spoke to the matter quite eloquently:
For that which he has not taken up he has not saved. He saved that which he joined to his divinity. If only half of Adam had fallen, then it would be possible for Christ to take up and save only half. But if the entire human nature fell, all of it must be united to the Word in order to be saved as a whole.Apollinarianism was rejected by the First Council of Constantinople in A.D. 381. This was due, in part, to the arguments of Theodore of Mopsuestia of the Antiochene School. Theodore argued against the view advocated by Apollinaris by stating that his position contradicted Scripture. The New Testament describes Christ as "growing in wisdom." If Jesus' mind was only divine, then Apollinaris would have to conclude that the divine mind was growing in wisdom. Did Apollinaris want to say that?
Theodore had to do more than put the inadequacies of Apollinaris' position on display. He had to offer an alternative. He stated that Christ had two natures in one person. He said that Christ was fully human. He did not simply have a human body, but felt pain and emotion as well. Since divinity was not subject to change, Christ's divine nature did not experience emotion. Theodore's position can be understood in this way: the two natures of Christ are two subjects to which are assigned different predicates. When Christ wept or feared, that was his human nature. When he performed miracles or forgave sins, that was his divine nature. The problem with Theodore's argument is that it, for all practical purposes, created a schizophrenic Jesus, who acted as one person in one instance, and another at a later time.