Thomas Aquinas

“ For since human nature is known to men only as it is subject to these defects, if the Son of God had assumed human nature without these defects, He would not have seemed to be true man, nor to have true, but imaginary, flesh, as the Manicheans held. ”

“ The fulness of all grace and knowledge was due to Christ's soul of itself, from the fact of its being assumed by the Word of God; and hence Christ assumed all the fulness of knowledge and wisdom absolutely. But He assumed our defects economically, in order to satisfy for our sin, and not that they belonged to Him of Himself. Hence it was not necessary for Him to assume them all, but only such as sufficed to satisfy for the sin of the whole nature.
Reply to Objection 3: Death comes to all men from the sin of our first parent; but not other defects, although they are less than death. ”

“ Now one satisfies for another's sin by taking on himself the punishment due to the sin of the other. But these bodily defects, to wit, death, hunger, thirst, and the like, are the punishment of sin, which was brought into the world by Adam, according to Rom. 5:12: "By one man sin entered into this world, and by sin death." Hence it was useful for the end of the Incarnation that He should assume these penalties in our flesh and in our stead, according to Is. 53:4, "Surely He hath borne our infirmities." ”