Definition: A formulation of the relationship between Jesus’ divinity and Jesus’ humanity.
More about the term: The hupostasis is the two natures in one Person, being Jesus Christ. Hupo means under, and stasis refers to substance. Though the English words “nature” and “substance” can be synonymous, meaning essence, we need to make a distinction for theological purposes. If nature is conceived of as a substantive entity, then nature and substance would be the same, and the incarnate Christ would consist of two substances, and would be two Persons (Nestorianism). But if “nature” is viewed as a “complex of attributes” this error is more apt to be avoided. The single Person of the incarnate Christ retained the total complex of divine attributes and possessed all the complex of human attributes essential to a perfect human being.
Here we must remedy Apollinarius’ thought on the Logos. Apollinarius believed that the Logos did not possess what would qualify as a human rational soul. On the contrary, the Logos, in consistence with the Imago Dei, does possess the attributes of a human rational soul. If the Logos does not posses the attributes, it compromises the Imago Dei and then we would not be made completely in the Image of God.
What makes a human a human? —A physical body and a rational soul.
What makes God to be God? —Divine necessity and divine attributes.
So how can He be ignorant if He is God? How can He not sin if He is human? Is there a polarity here?
No, once there is a rational conception of the two natures in one Being, it fits like a puzzle. Though these questions are not all the questions there may be, they are examples of how the two natures function in a rational compatibility. Self-consciousness may play a role. The question is whether Christ in His own self-consciousness was aware of His deity and humanity. The answer is that the Person was always aware in Himself to His deity and that the Person grew in self-consciousness with respect to His humanity.
Jesus is able to be sinless because although the Bible teaches that everyone has sinned (Rom. 3:23), sin is not necessary. Recall the possibility of a world that does and does not have sin (used in the problem of evil):
There is a possible world in which all free creatures willingly and freely choose to do right.
There is a possible world in which all free creatures willingly and freely choose to do evil.
Thus, it is possible that every world God could create containing free creatures would be a world with sin and evil.
This does not mean that Jesus is created (outside of the biological human complexity that exists in the physical body). This is in relation to Jesus as a free agent, even more so due to humans having the notion permission. The two natures exist eternally and are not created (Imago Dei). In this logic, we can see that human righteousness is not dependent upon sin (just as we have a rational soul in our human nature, we can choose to do right without necessity of wrong). So it is possible for Jesus to be genuinely tempted (in His human nature), while still maintaining His divine nature. Jesus’ human nature was able to feel the draw and lure of temptation but would not be able to sin because of His divine nature. God cannot feel the draw and lure of temptation, thus it was His human nature that was tempted. To sum this point up, righteousness is not contingent upon sin.
 This point disagrees with the Fifth Ecumenical Council, which affirmed diothelitism (two wills of Christ). I would tend to believe there to be only one will, otherwise there would be a duality of two persons (Nestorianism). There was a full human mind and intellect with a rational soul.