4 On a second night, however, the same youth appeared to Gennadius, and asked whether he recognised him, to which he replied that he knew him well, without the slightest uncertainty. Thereupon he asked Gennadius where he had become acquainted with him. There also his memory failed him not as to the proper reply: he narrated the whole vision, and the hymns of the saints which, under his guidance, he had been taken to hear, with all the readiness natural to recollection of some very recent experience.
On this the youth inquired whether it was in sleep or when awake that he had seen what he had just narrated. Gennadius answered: “In sleep.” The youth then said: “You remember it well; it is true that you saw these things in sleep, but I would have you know that even now you are seeing in sleep.” Hearing this, Gennadius was persuaded of its truth, and in his reply declared that he believed it. Then his teacher went on to say: “Where is your body now?” He answered: “In my bed.” “Do you know,” said the youth, “that the eyes in this body of yours are now bound and closed, and at rest, and that with these eyes you are seeing nothing?”
He answered: “I know it.” “What, then,” said the youth, “are the eyes with which you see me?” He, unable to discover what to answer to this, was silent. While he hesitated, the youth unfolded to him what he was endeavoring to teach him by these questions, and immediately said: “As while you are asleep and lying on your bed these eyes of your body are now unemployed and doing nothing, and yet you have eyes with which you behold me, and enjoy this vision, so, after your death, while your bodily eyes shall be wholly inactive, there shall be in you a life by which you shall still live, and a faculty of perception by which you shall still perceive.
Beware, therefore, after this of harbouring doubts as to whether the life of man shall continue after death.” This believer says that by this means all doubts as to this matter were removed from him. By whom was he taught this but by the merciful, providential care of God?
Source: Letters (New Advent)