John Cassian, Conferences I–X X.9: The answer on the efficacy of understanding, which is gained by experience

Isaac: Your minute and subtle inquiry affords an indication of purity being very nearly reached. For no one would be able even to make inquiries on these matters, I will not say to look within and discriminate,—except one who had been urged to sound the depths of such questions by careful and effectual diligence of mind, and watchful anxiety, and one whom the constant aim after a well controlled life had taught by practical experience to attempt the entrance to this purity and to knock at its doors. And therefore as I see you, I will not say, standing before the doors of that true prayer of which we have been speaking, but touching its inner chambers and inward parts as it were with the hands of experience, and already laying hold of some parts of it, I do not think that I shall find any difficulty in introducing you now within what I may call its hall, for you to roam about its recesses, as the Lord may direct; nor do I think that you will be hindered from investigating what is to be shown you by any obstacles or difficulties. For he is next door to understanding who carefully recognizes what he ought to ask about, nor is he far from knowledge, who begins to understand how ignorant he is. And therefore I am not afraid of the charge of betraying secrets, and of levity, if I divulge what when speaking in my former discourse on the perfection of prayer I had kept back from discussing, as I think that its force was to be explained to us who are occupied with this subject and interest even without the aid of my words, by the grace of God.