19 Hence too is solved that question, how is it that the Martyrs, by the very benefits which are given to them that pray, indicate that they take an interest in the affairs of men, if the dead know not what the quick are doing. For not only by effects of benefits, but in the very beholding of men, it is certain, that the Confessor Felix (whose denizenship among you you piously love) appeared when the barbarians were attacking Nola, as we have heard not by uncertain rumors, but by sure witnesses.
But such things are of God exhibited, far otherwise than as the usual order has itself, unto each kind of creatures apportioned. For it does not follow because water was, when it pleased the Lord, in a moment changed into wine, that we are not to regard the worth and efficacy of water in the proper order of the elements, as distinct from the rarity, or rather singularity, of that divine work: nor because Lazarus rose again, therefore that every dead man rises when he will; or that a lifeless man is raised up by a living, in the same way as a sleeping man by one who is awake.
Other be the limits of human things, other the signs of divine virtues: other they be that are naturally, other that be miraculously done: albeit both unto nature God is present that it may be, and unto miracles nature is not lacking. We are not to think then, that to be interested in the affairs of the living is in the power of any departed who please, only because to some men's healing or help the Martyrs be present: but rather we are to understand that it must needs be by a Divine power that the Martyrs are interested in affairs of the living, from the very fact that for the departed to be by their proper nature interested in affairs of the living is impossible.
Source: On Care to be Had For the Dead (New Advent)