Objection 1: It would seem that the Son of God ought not to have assumed human nature with defects of body. For as His soul is personally united to the Word of God, so also is His body. But the soul of Christ had every perfection, both of grace and truth, as was said above (Q. 7, A. 9; Q. 9, seqq.). Hence, His body also ought to have been in every way perfect, not having any imperfection in it.
Obj. 2: Further, the soul of Christ saw the Word of God by the vision wherein the blessed see, as was said above (Q. 9, A. 2), and thus the soul of Christ was blessed. Now by the beatification of the soul the body is glorified; since, as Augustine says (Ep. ad Dios. cxviii), "God made the soul of a nature so strong that from the fulness of its blessedness there pours over even into the lower nature" (i.e. the body), "not indeed the bliss proper to the beatific fruition and vision, but the fulness of health" (i.e. the vigor of incorruptibility). Therefore the body of Christ was incorruptible and without any defect.
Obj. 3: Further, penalty is the consequence of fault. But there was no fault in Christ, according to 1 Pet. 2:22: "Who did no guile." Therefore defects of body, which are penalties, ought not to have been in Him.
Obj. 4: Further, no reasonable man assumes what keeps him from his proper end. But by such like bodily defects, the end of the Incarnation seems to be hindered in many ways. First, because by these infirmities men were kept back from knowing Him, according to Isa. 53:2, 3: "[There was no sightliness] that we should be desirous of Him. Despised and the most abject of men, a man of sorrows and acquainted with infirmity, and His look was, as it were, hidden and despised, whereupon we esteemed Him not." Secondly, because the desire of the Fathers would not seem to be fulfilled, in whose person it is written (Isa. 51:9): "Arise, arise, put on Thy strength, O Thou Arm of the Lord." Thirdly, because it would seem more fitting for the devil's power to be overcome and man's weakness healed, by strength than by weakness. Therefore it does not seem to have been fitting that the Son of God assumed human nature with infirmities or defects of body.
On the contrary, It is written (Heb. 2:18): "For in that, wherein He Himself hath suffered and been tempted, He is able to succor them also that are tempted." Now He came to succor us. Hence David said of Him (Ps. 120:1): "I have lifted up my eyes to the mountains, from whence help shall come to me." Therefore it was fitting for the Son of God to assume flesh subject to human infirmities, in order to suffer and be tempted in it and so bring succor to us.
I answer that, It was fitting for the body assumed by the Son of God to be subject to human infirmities and defects; and especially for three reasons. First, because it was in order to satisfy for the sin of the human race that the Son of God, having taken flesh, came into the world. 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 Isa. 53:4, "Surely He hath borne our infirmities." Secondly, in order to cause belief in the Incarnation. 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. And so, as is said, Phil. 2:7: "He . . . emptied Himself, taking the form of a servant, being made in the likeness of men, and in habit found as a man." Hence, Thomas, by the sight of His wounds, was recalled to the faith, as related John 20:26. Thirdly, in order to show us an example of patience by valiantly bearing up against human passibility and defects. Hence it is said (Heb. 12:3) that He "endured such opposition from sinners against Himself, that you be not wearied, fainting in your minds."
Reply Obj. 1: The penalties one suffers for another's sin are the matter, as it were, of the satisfaction for that sin; but the principle is the habit of soul, whereby one is inclined to wish to satisfy for another, and from which the satisfaction has its efficacy, for satisfaction would not be efficacious unless it proceeded from charity, as will be explained (Supp., Q. 14, A. 2). Hence, it behooved the soul of Christ to be perfect as regards the habit of knowledge and virtue, in order to have the power of satisfying; but His body was subject to infirmities, that the matter of satisfaction should not be wanting.
Reply Obj. 2: From the natural relationship which is between the soul and the body, glory flows into the body from the soul's glory. Yet this natural relationship in Christ was subject to the will of His Godhead, and thereby it came to pass that the beatitude remained in the soul, and did not flow into the body; but the flesh suffered what belongs to a passible nature; thus Damascene says (De Fide Orth. iii, 15) that, "it was by the consent of the Divine will that the flesh was allowed to suffer and do what belonged to it."
Reply Obj. 3: Punishment always follows sin actual or original, sometimes of the one punished, sometimes of the one for whom he who suffers the punishment satisfies. And so it was with Christ, according to Isa. 53:5: "He was wounded for our iniquities, He was bruised for our sins."
Reply Obj. 4: The infirmity assumed by Christ did not impede, but greatly furthered the end of the Incarnation, as above stated. And although these infirmities concealed His Godhead, they made known His Manhood, which is the way of coming to the Godhead, according to Rom. 5:1, 2: "By Jesus Christ we have access to God." Moreover, the ancient Fathers did not desire bodily strength in Christ, but spiritual strength, wherewith He vanquished the devil and healed human weakness.