Objection 1. It would seem that paradise was not a place adapted to be the abode of man. For man and angels are similarly ordered to beatitude. But the angels from the very beginning of their existence were made to dwell in the abode of the blessed--that is, the empyrean heaven. Therefore the place of man's habitation should have been there also.
Objection 2. Further, if some definite place were required for man's abode, this would be required on the part either of the soul or of the body. If on the part of the soul, the place would be in heaven, which is adapted to the nature of the soul; since the desire of heaven is implanted in all. On the part of the body, there was no need for any other place than the one provided for other animals. Therefore paradise was not at all adapted to be the abode of man.
Objection 3. Further, a place which contains nothing is useless. But after sin, paradise was not occupied by man. Therefore if it were adapted as a dwelling-place for man, it seems that God made paradise to no purpose.
Objection 4. Further, since man is of an even temperament, a fitting place for him should be of even temperature. But paradise was not of an even temperature; for it is said to have been on the equator--a situation of extreme heat, since twice in the year the sun passes vertically over the heads of its inhabitants. Therefore paradise was not a fit dwelling-place for man.
On the contrary, Damascene says (De Fide Orth. ii, 11): "Paradise was a divinely ordered region, and worthy of him who was made to God's image."
I answer that, As above stated (97, 1), Man was incorruptible and immortal, not because his body had a disposition to incorruptibility, but because in his soul there was a power preserving the body from corruption. Now the human body may be corrupted from within or from without. From within, the body is corrupted by the consumption of the humors, and by old age, as above explained (97, 4), and man was able to ward off such corruption by food. Among those things which corrupt the body from without, the chief seems to be an atmosphere of unequal temperature; and to such corruption a remedy is found in an atmosphere of equable nature. In paradise both conditions were found; because, as Damascene says (De Fide Orth. ii, 11): "Paradise was permeated with the all pervading brightness of a temperate, pure, and exquisite atmosphere, and decked with ever-flowering plants." Whence it is clear that paradise was most fit to be a dwelling-place for man, and in keeping with his original state of immortality.
Reply to Objection 1. The empyrean heaven is the highest of corporeal places, and is outside the region of change. By the first of these two conditions, it is a fitting abode for the angelic nature: for, as Augustine says (De Trin. ii), "God rules corporeal creatures through spiritual creatures." Hence it is fitting that the spiritual nature should be established above the entire corporeal nature, as presiding over it. By the second condition, it is a fitting abode for the state of beatitude, which is endowed with the highest degree of stability. Thus the abode of beatitude was suited to the very nature of the angel; therefore he was created there. But it is not suited to man's nature, since man is not set as a ruler over the entire corporeal creation: it is a fitting abode for man in regard only to his beatitude. Wherefore he was not placed from the beginning in the empyrean heaven, but was destined to be transferred thither in the state of his final beatitude.
Reply to Objection 2. It is ridiculous to assert that any particular place is natural to the soul or to any spiritual substances, though some particular place may have a certain fitness in regard to spiritual substances. For the earthly paradise was a place adapted to man, as regards both his body and his soul--that is, inasmuch as in his soul was the force which preserved the human body from corruption. This could not be said of the other animals. Therefore, as Damascene says (De Fide Orth. ii, 11): "No irrational animal inhabited paradise"; although, by a certain dispensation, the animals were brought thither by God to Adam; and the serpent was able to trespass therein by the complicity of the devil.
Reply to Objection 3. Paradise did not become useless through being unoccupied by man after sin, just as immortality was not conferred on man in vain, though he was to lose it. For thereby we learn God's kindness to man, and what man lost by sin. Moreover, some say that Enoch and Elias still dwell in that paradise.
Reply to Objection 4. Those who say that paradise was on the equinoctial line are of opinion that such a situation is most temperate, on account of the unvarying equality of day and night; that it is never too cold there, because the sun is never too far off; and never too hot, because, although the sun passes over the heads of the inhabitants, it does not remain long in that position. However, Aristotle distinctly says (Meteor. ii, 5) that such a region is uninhabitable on account of the heat. This seems to be more probable; because, even those regions where the sun does not pass vertically overhead, are extremely hot on account of the mere proximity of the sun. But whatever be the truth of the matter, we must hold that paradise was situated in a most temperate situation, whether on the equator or elsewhere.
Source: Summa Theologica (New Advent)