According to the theory held by the ancient Jews and by the whole of the Semitic nations, everything of real value that from time to time appears on earth has its existence in heaven. In other words, it exists with God, that is God possesses a knowledge of it; and for that reason it has a real being. But it exists beforehand with God in the same way as it appears on earth, that is with all the material attributes belonging to its essence. Its manifestation on earth is merely a transition from concealment to publicity (φανεροu,σθαι). In becoming visible to the senses, the object in question assumes no attribute that it did not already possess with God. Hence its material nature is by no means an inadequate expression of it, nor is it a second nature added to the first. The truth rather is that what was in heaven before is now revealing itself upon earth, without any sort of alteration taking place in the process. There is no assumptio naturæ novæ, and no change or mixture. The old Jewish theory of pre-existence is founded on the religious idea of the omniscience and omni-potence of God, that God to whom the events of history do not come as a surprise, but who guides their course. As the whole history of the world and the destiny of each individual are recorded on his tablets or books, so also each thing is ever present before him. The decisive contrast is between God and the creature. In designating the latter as “foreknown” by God, the primary idea is not to ennoble the creature, but rather to bring to light the wisdom and power of God. The ennobling of created things by attributing to them a pre-existence is a secondary result (see below).
According to the Hellenic conception, which has become associated with Platonism, the idea of pre-existence is independent of the idea of God; it is based on the conception of the contrast between spirit and matter, between the infinite and finite, found in the cosmos itself. In the case of all spiritual beings, life in the body or flesh is at bottom an inadequate and unsuitable condition, for the spirit is eternal, the flesh perishable. But the pre-temporal existence, which was only a doubtful assumption as regards ordinary spirits, was a matter of certainty in the case of the higher and purer ones. They lived in an upper world long before this earth was created, and they lived there as spirits without the “polluted garment of the flesh.” Now if they resolved for some reason or other to appear in this finite world, they cannot simply become visible, for they have no “visible form.” They must rather “assume flesh,” whether they throw it about them as a covering, or really make it their own by a process of transformation or mixture. In all cases—and here the speculation gave rise to the most exciting problems—the body is to them something inadequate which they cannot appropriate without adopting certain measures of precaution, but this process may indeed pass through all stages, from a mere seeming appropriation to complete union. The characteristics of the Greek ideas of pre-existence may consequently be thus expressed. First, the objects in question to which pre-existence is ascribed are meant to be ennobled by this attribute. Secondly, these ideas have no relation to God. Thirdly, the material appearance is regarded as something inadequate. Fourthly, speculations about phantasma, assumptio naturæ humanæ, transmutatio, mixtura, duæ naturæ, etc., were necessarily associated with these notions.
We see that these two conceptions are as wide apart as the poles. The first has a religious origin, the second a cosmological and psychological; the first glorifies God, the second the created spirit.
from page 318, History of Dogma