Augustine, On the Lord's Sermon on the Mount 2.3.14 (Latin text available):

`But again one might ask whether we are to pray by words or deeds and what need there is for prayer, if God already nows what is needful for us. But it is because the act of prayer clarifies and purges our heart and makes it more capable of receiving the divine gifts that are poured out for us in the spirit. God does not give heed to the ambitiousness of our prayers, because he is always ready to give to us his light, not a visible light but an intellectual and spiritual one: but we are not always read to receive it when we turn aside and down to other things out of a desire for temporal things. For in prayer there occurs a turning of the heart to he who is always ready to give if we will but take what he gives: and in that turning is the purification of the inner eye when the things we crave in the temporal world are shut out; so that the vision of the pure heart can bear the pure light that shines divinely without setting or wavering: and not only bear it, but abide in it; not only without difficulty, but even with unspeakable joy, with which the blessed life is truly and genuinely brought to fulfillment.'