Thomas Aquinas, Compendium of Theology

We saw above that man was originally constituted by God in such a condition that his body was completely subject to his soul. Further, among the faculties of the soul, the lower powers were subject to reason without any rebelliousness, and man's reason itself was subject to God. In consequence of the perfect subjection of the body to the soul, no passion could arise in the body that would in any way conflict with the soul's dominion over the body. Therefore neither death nor illness had any place in man. And from the subjection of the lower powers to reason there resulted in man complete peace of mind, for the human reason was troubled by no inordinate passions. Finally, owing to the submission of man's will to God, man referred all things to God as to his last end, and in this his justice and innocence consisted.

Of these three subordinations, the last was the cause of the other two. Surely man's freedom from dissolution or from any suffering that would be a threat to his life, did not come from the nature of his body, as we see if we regard its component parts; for the body was made up of contrary elements. Similarly, the fact that man's sense faculties were subservient to reason without any rebelliousness did not come from the nature of the soul, since the sense powers naturally tend toward objects that cause pleasure in the senses, even when, as often happens, delights of this sort are at odds with right reason.

This harmony came from a higher power, the power of God. It was God who, in the first instance, united to the body the rational soul that so immeasurably surpasses the body and the bodily faculties, such as the sense powers. Likewise it was God who gave to the rational soul the power to control the body itself in a manner that exceeded the natural condition of the body, and also to govern the sense faculties so that they would function in a way befitting a rational soul. In order, therefore, that reason might firmly hold the lower faculties under its sway, reason itself had to be firmly kept under the dominion of God, from whom it received this power so greatly surpassing the condition of nature. Accordingly man was so constituted that, unless his reason was subservient to God, his body could not be made subject to the beck of the soul, nor his sense powers be brought under the rule of reason. Hence in that state life was in a certain way immortal and impassible; that is, man could neither die nor suffer, so long as he did not sin. Nevertheless he retained the power to sin, since his will was not yet confirmed in good by the attainment of the last end; in the event that this happened, man could suffer and die.

It is precisely in this respect that the impassibility and immortality possessed by the first man differ from the impassibility and immortality to be enjoyed after the resurrection by the saints, who will never be subject to suffering and death, since their wills will be wholly fixed upon God, as we said above. There is another difference: after the resurrection men will have no use for food or the reproductive functions; but the first man was so constituted that he had to sustain his life with food, and he had a mandate to perform the work of generation; for the human race was to be multiplied from this one parent. Hence he received two commands, in keeping with his condition. The first is that mentioned in Genesis 2:16: "Of every tree of Paradise thou shalt eat." The other is reported in Genesis 1:28: "Increase and multiply and fill the earth."