Objection 1. It would seem that in the state of innocence children would have had perfect strength of the body, as to the use of its members, immediately after birth. For Augustine says (De Pecc. Merit. et Remiss. i, 38): "This weakness of the body befits their weakness of mind." But in the state of innocence there would have been no weakness of mind. Therefore neither would there have been weakness of body in infants.
Objection 2. Further, some animals at birth have sufficient strength to use their members. But man is nobler than other animals. Therefore much more is it natural to man to have strength to use his members at birth; and thus it appears to be a punishment of sin that he has not that strength.
Objection 3. Further, inability to secure a proffered pleasure causes affliction. But if children had not full strength in the use of their limbs, they would often have been unable to procure something pleasurable offered to them; and so they would have been afflicted, which was not possible before sin. Therefore, in the state of innocence, children would not have been deprived of the use of their limbs.
Objection 4. Further, the weakness of old age seems to correspond to that of infancy. But in the state of innocence there would have been no weakness of old age. Therefore neither would there have been such weakness in infancy.
On the contrary, Everything generated is first imperfect. But in the state of innocence children would have been begotten by generation. Therefore from the first they would have been imperfect in bodily size and power.
I answer that, By faith alone do we hold truths which are above nature, and what we believe rests on authority. Wherefore, in making any assertion, we must be guided by the nature of things, except in those things which are above nature, and are made known to us by Divine authority. Now it is clear that it is as natural as it is befitting to the principles of human nature that children should not have sufficient strength for the use of their limbs immediately after birth. Because in proportion to other animals man has naturally a larger brain. Wherefore it is natural, on account of the considerable humidity of the brain in children, that the nerves which are instruments of movement, should not be apt for moving the limbs. On the other hand, no Catholic doubts it possible for a child to have, by Divine power, the use of its limbs immediately after birth.
Now we have it on the authority of Scripture that "God made man right" (Ecclesiastes 7:30), which rightness, as Augustine says (De Civ. Dei xiv, 11), consists in the perfect subjection of the body to the soul. As, therefore, in the primitive state it was impossible to find in the human limbs anything repugnant to man's well-ordered will, so was it impossible for those limbs to fail in executing the will's commands. Now the human will is well ordered when it tends to acts which are befitting to man. But the same acts are not befitting to man at every season of life. We must, therefore, conclude that children would not have had sufficient strength for the use of their limbs for the purpose of performing every kind of act; but only for the acts befitting the state of infancy, such as suckling, and the like.
Reply to Objection 1. Augustine is speaking of the weakness which we observe in children even as regards those acts which befit the state of infancy; as is clear from his preceding remark that "even when close to the breast, and longing for it, they are more apt to cry than to suckle."
Reply to Objection 2. The fact that some animals have the use of their limbs immediately after birth, is due, not to their superiority, since more perfect animals are not so endowed; but to the dryness of the brain, and to the operations proper to such animals being imperfect, so that a small amount of strength suffices them.
Reply to Objection 3 is clear from what we have said above. We may add that they would have desired nothing except with an ordinate will; and only what was befitting to their state of life.
Reply to Objection 4. In the state of innocence man would have been born, yet not subject to corruption. Therefore in that state there could have been certain infantile defects which result from birth; but not senile defects leading to corruption.
Source: Summa Theologica (New Advent)