Given that the account of the creation and fall of man is not in a historical genre, what does the account intend to say about the world?

There are two ways in which an account like this can be taken. In one way, it would simply be a guess about the nature and origins of humanity. Taken in this way, it would present a possibility, but not claim that it is definitely the case.

The Catholic Church understands the text in a second way, namely as presenting the same possibility, but also as claiming that this possibility is actual. Thus the Catechism states,

The account of the fall in Genesis 3 uses figurative language, but affirms a primeval event, a deed that took place at the beginning of the history of man. Revelation gives us the certainty of faith that the whole of human history is marked by the original fault freely committed by our first parents.

This is a reasonable understanding of Genesis, even the first way is possible in principle. Taken as the Catechism takes it, Genesis would be saying that humanity existed in a more perfect state, and fell to a less perfect state due to human sin. St. Paul seems to have a similar understanding of Genesis, saying that “sin came into the world through one man.”

The text of Genesis, which should be understood as an image, presents this more perfect state as a situation where Adam and Eve are in the garden of Eden, living off the fruit of the trees, naked and unashamed, and with access to the tree of life, by which they might live forever.

The Catechism does not seem to summarize the more perfect state, but describes it indirectly, by contrasting it with the fallen state:

The harmony in which they had found themselves, thanks to original justice, is now destroyed: the control of the soul’s spiritual faculties over the body is shattered; the union of man and woman becomes subject to tensions, their relations henceforth marked by lust and domination. Harmony with creation is broken: visible creation has become alien and hostile to man. Because of man, creation is now subject “to its bondage to decay”. Finally, the consequence explicitly foretold for this disobedience will come true: man will “return to the ground”, for out of it he was taken. Death makes its entrance into human history.

St. Thomas discusses whether the defects that resulted from sin are natural or not:

Objection 1. It would seem that death and such like defects are natural to man. For “the corruptible and the incorruptible differ generically” (Metaph. x, text. 26). But man is of the same genus as other animals which are naturally corruptible. Therefore man is naturally corruptible.

Objection 2. Further, whatever is composed of contraries is naturally corruptible, as having within itself the cause of corruption. But such is the human body. Therefore it is naturally corruptible.

Objection 3. Further, a hot thing naturally consumes moisture. Now human life is preserved by hot and moist elements. Since therefore the vital functions are fulfilled by the action of natural heat, as stated in De Anima ii, text. 50, it seems that death and such like defects are natural to man.

On the contrary, (1) God made in man whatever is natural to him. Now “God made not death” (Wisdom 1:13). Therefore death is not natural to man.

(2) Further, that which is natural cannot be called either a punishment or an evil: since what is natural to a thing is suitable to it. But death and such like defects are the punishment of original sin, as stated above (Article 5). Therefore they are not natural to man.

(3) Further, matter is proportionate to form, and everything to its end. Now man’s end is everlasting happiness, as stated above (2, 7; 5, A3,4): and the form of the human body is the rational soul, as was proved in the I, 75, 6. Therefore the human body is naturally incorruptible.

I answer that, We may speak of any corruptible thing in two ways; first, in respect of its universal nature, secondly, as regards its particular nature. A thing’s particular nature is its own power of action and self-preservation. And in respect of this nature, every corruption and defect is contrary to nature, as stated in De Coelo ii, text. 37, since this power tends to the being and preservation of the thing to which it belongs.

On the other hand, the universal nature is an active force in some universal principle of nature, for instance in some heavenly body; or again belonging to some superior substance, in which sense God is said by some to be “the Nature Who makes nature.” This force intends the good and the preservation of the universe, for which alternate generation and corruption in things are requisite: and in this respect corruption and defect in things are natural, not indeed as regards the inclination of the form which is the principle of being and perfection, but as regards the inclination of matter which is allotted proportionately to its particular form according to the discretion of the universal agent. And although every form intends perpetual being as far as it can, yet no form of a corruptible being can achieve its own perpetuity, except the rational soul; for the reason that the latter is not entirely subject to matter, as other forms are; indeed it has an immaterial operation of its own, as stated in the I, 75, 2. Consequently as regards his form, incorruption is more natural to man than to other corruptible things. But since that very form has a matter composed of contraries, from the inclination of that matter there results corruptibility in the whole. In this respect man is naturally corruptible as regards the nature of his matter left to itself, but not as regards the nature of his form.

The first three objections argue on the side of the matter; while the other three argue on the side of the form. Wherefore in order to solve them, we must observe that the form of man which is the rational soul, in respect of its incorruptibility is adapted to its end, which is everlasting happiness: whereas the human body, which is corruptible, considered in respect of its nature, is, in a way, adapted to its form, and, in another way, it is not. For we may note a twofold condition in any matter, one which the agent chooses, and another which is not chosen by the agent, and is a natural condition of matter. Thus, a smith in order to make a knife, chooses a matter both hard and flexible, which can be sharpened so as to be useful for cutting, and in respect of this condition iron is a matter adapted for a knife: but that iron be breakable and inclined to rust, results from the natural disposition of iron, nor does the workman choose this in the iron, indeed he would do without it if he could: wherefore this disposition of matter is not adapted to the workman’s intention, nor to the purpose of his art. In like manner the human body is the matter chosen by nature in respect of its being of a mixed temperament, in order that it may be most suitable as an organ of touch and of the other sensitive and motive powers. Whereas the fact that it is corruptible is due to a condition of matter, and is not chosen by nature: indeed nature would choose an incorruptible matter if it could. But God, to Whom every nature is subject, in forming man supplied the defect of nature, and by the gift of original justice, gave the body a certain incorruptibility, as was stated in the I, 97, 1. It is in this sense that it is said that “God made not death,” and that death is the punishment of sin.

This suffices for the Replies to the Objections.

Basically St. Thomas is asserting that the condition of mankind was not natural in the sense of caused by nature, but it was something that would be desirable to nature. After sin, man fell basically to the level that could be caused by nature.

St. Thomas thus suggests a reason why it was fitting for man to be created in an immortal condition. He says in the cited article:

Thirdly, a thing may be incorruptible on the part of its efficient cause; in this sense man was incorruptible and immortal in the state of innocence. For, as Augustine says (QQ. Vet. et Nov. Test. qu. 19): “God made man immortal as long as he did not sin; so that he might achieve for himself life or death.” For man’s body was indissoluble not by reason of any intrinsic vigor of immortality, but by reason of a supernatural force given by God to the soul, whereby it was enabled to preserve the body from all corruption so long as it remained itself subject to God. This entirely agrees with reason; for since the rational soul surpasses the capacity of corporeal matter, as above explained (76, 1), it was most properly endowed at the beginning with the power of preserving the body in a manner surpassing the capacity of corporeal matter.

Understood in this way, the account in Genesis asserts at least two things: that man was created in a state above the state that could be caused by nature, because this was better, and that man fell from this state due to his sin.