Objection 1. It would seem that paradise is not a corporeal place. For Bede [Strabus, Gloss on Genesis 2:8] says that "paradise reaches to the lunar circle." But no earthly place answers that description, both because it is contrary to the nature of the earth to be raised up so high, and because beneath the moon is the region of fire, which would consume the earth. Therefore paradise is not a corporeal place.
Objection 2. Further, Scripture mentions four rivers as rising in paradise (Genesis 2:10). But the rivers there mentioned have visible sources elsewhere, as is clear from the Philosopher (Meteor. i). Therefore paradise is not a corporeal place.
Objection 3. Further, although men have explored the entire habitable world, yet none have made mention of the place of paradise. Therefore apparently it is not a corporeal place.
Objection 4. Further, the tree of life is described as growing in paradise. But the tree of life is a spiritual thing, for it is written of Wisdom that "She is a tree of life to them that lay hold on her" (Proverbs 3:18). Therefore paradise also is not a corporeal, but a spiritual place.
Objection 5. Further, if paradise be a corporeal place, the trees also of paradise must be corporeal. But it seems they were not; for corporeal trees were produced on the third day, while the planting of the trees of paradise is recorded after the work of the six days. Therefore paradise was not a corporeal place.
On the contrary, Augustine says (Gen. ad lit. viii, 1): "Three general opinions prevail about paradise. Some understand a place merely corporeal; others a place entirely spiritual; while others, whose opinion, I confess, hold that paradise was both corporeal and spiritual."
I answer that, As Augustine says (De Civ. Dei xiii, 21): "Nothing prevents us from holding, within proper limits, a spiritual paradise; so long as we believe in the truth of the events narrated as having there occurred." For whatever Scripture tells us about paradise is set down as matter of history; and wherever Scripture makes use of this method, we must hold to the historical truth of the narrative as a foundation of whatever spiritual explanation we may offer. And so paradise, as Isidore says (Etym. xiv, 3), "is a place situated in the east, its name being the Greek for garden." It was fitting that it should be in the east; for it is to be believed that it was situated in the most excellent part of the earth. Now the east is the right hand on the heavens, as the Philosopher explains (De Coel. ii, 2); and the right hand is nobler than the left: hence it was fitting that God should place the earthly paradise in the east.
Reply to Objection 1. Bede's assertion is untrue, if taken in its obvious sense. It may, however, be explained to mean that paradise reaches to the moon, not literally, but figuratively; because, as Isidore says (Etym. xiv, 3), the atmosphere there is "a continually even temperature"; and in this respect it is like the heavenly bodies, which are devoid of opposing elements. Mention, however, is made of the moon rather than of other bodies, because, of all the heavenly bodies, the moon is nearest to us, and is, moreover, the most akin to the earth; hence it is observed to be overshadowed by clouds so as to be almost obscured. Others say that paradise reached to the moon--that is, to the middle space of the air, where rain, and wind, and the like arise; because the moon is said to have influence on such changes. But in this sense it would not be a fit place for human dwelling, through being uneven in temperature, and not attuned to the human temperament, as is the lower atmosphere in the neighborhood of the earth.
Reply to Objection 2. Augustine says (Gen. ad lit. viii, 7): "It is probable that man has no idea where paradise was, and that the rivers, whose sources are said to be known, flowed for some distance underground, and then sprang up elsewhere. For who is not aware that such is the case with some other streams?"
Reply to Objection 3. The situation of paradise is shut off from the habitable world by mountains, or seas, or some torrid region, which cannot be crossed; and so people who have written about topography make no mention of it.
Reply to Objection 4. The tree of life is a material tree, and so called because its fruit was endowed with a life-preserving power as above stated (97, 4). Yet it had a spiritual signification; as the rock in the desert was of a material nature, and yet signified Christ. In like manner the tree of the knowledge of good and evil was a material tree, so called in view of future events; because, after eating of it, man was to learn, by experience of the consequent punishment, the difference between the good of obedience and the evil of rebellion. It may also be said to signify spiritually the free-will as some say.
Reply to Objection 5. According to Augustine (Gen. ad lit. v, 5, viii, 3), the plants were not actually produced on the third day, but in their seminal virtues; whereas, after the work of the six days, the plants, both of paradise and others, were actually produced. According to other holy writers, we ought to say that all the plants were actually produced on the third day, including the trees of paradise; and what is said of the trees of paradise being planted after the work of the six days is to be understood, they say, by way of recapitulation. Whence our text reads: "The Lord God had planted a paradise of pleasure from the beginning" (Genesis 2:8).
Source: Summa Theologica (New Advent)