Nicene and Post-Nicene Fathers, Ser. II, Vol. XI:
The Works of John Cassian.: Chapter III. Our answer on the fickle character of our thoughts.

Chapter III.

Our answer on the fickle character of our thoughts.

The time spent here, and the dwelling in solitude, and meditation, through which you think that we ought to have attained perfection of the inner man, has only done this for us; viz., teach us that which we are unable to be, without making us what we are trying to be. Nor do we feel that by this knowledge we have acquired any fixed steadfastness of the purity which we long for, or any strength and firmness; but only an increase of confusion and shame: for though our meditation in all our discipline aims at this in our daily studies, and endeavours from trembling beginnings to reach a sure and unwavering skill, and to begin to know something of what originally it knew but vaguely or was altogether ignorant of, and by advancing by sure steps (so to speak) towards the condition of that discipline, to habituate itself perfectly to it without any difficulty, I find on the contrary that while I am struggling in this desire for purity, I have only got far enough to know what I cannot be. And hence I feel that nothing but trouble results to me from all this contrition of heart, so that matter for tears is never wanting, and yet I do not cease to be what I ought not to be. And so what is the good of having learnt what is best, if it cannot be attained even when known? for when we have been feeling that the aim of our heart was directed towards what we purposed, insensibly the mind returns to its previous wandering thoughts and slips back with a more violent rush, and is taken up with daily distractions and incessantly drawn away by numberless things that take it captive, so that we almost despair of the improvement which we long for, and all these observances seem useless. Since the mind which every moment wanders off vaguely, when it is brought back to the fear of God or spiritual contemplation, before it is established in it, darts off and strays; and when we have been roused and have discovered that it has wandered from the purpose set before it, and want to recall it to the meditation from which it has strayed, and to bind it fast with the firmest purpose of heart, as if with chains, while we are making the attempt it slips away from the inmost recesses of the heart swifter than a snake. Wherefore we being inflamed by daily exercises of this kind, and yet not seeing that we gain from them any strength and stability in heart are overcome and in despair driven to this opinion; viz., to believe that it is from no fault of our own but from a fault of our nature that these wanderings of mind are found in mankind.