When the spiritual internal is opened, and through it communication with heaven and conjunction with the Lord are granted, enlightenment takes place with man. He is enlightened especially when he reads the Word, because the Lord is in the Word, and the Word is Divine truth, and Divine truth is light to angels. Man is enlightened in the rational, for this directly underlies the spiritual internal, and receives light from heaven and transfers it into the natural when it is purified from evils, filling it with the knowledges of truth and good, and adapting to them the knowledges (scientiae) that are from world, for the sake of proof and agreement. Thus man has a rational, and thus he has an understanding. He who believes that man has a rational and an understanding before his natural has been purified from evils is deceived, for the understanding is seeing truths of the church from the light of heaven; and the light of heaven does not flow into those not purified. And as the understanding is perfected, the falsities of religion and of ignorance and all fallacies are dispersed.
When a man has been admitted by the opening of his internal into heaven, and receives light therefrom, the same affections that angels of heaven have, with their pleasures and delights, are communicated to him. The first affection then granted is an affection for truth; the second is an affection for good; and the third is an affection for bringing forth fruits. For when a man has been admitted into heaven and into its light and heat he is like a tree growing from its seed. His first budding forth is from enlightenment; his blossoming before the fruit is from an affection for truth; the putting forth of fruit that follows is from an affection for good; the multiplication of itself again into trees is from an affection for producing fruit. The heat of heaven, which is love, and the light of heaven, which is the understanding of truth from that love, bring forth in subjects of life things like those that the heat of the world and its light bring forth in subjects not of life. That like things are brought forth is from correspondence. But in both cases the production is effected in springtime; and springtime in man is when he enters heaven, which is effected when his spiritual internal is opened; before that it is the time of winter to him.
from Spiritual Life and the Word of God, Part first