Nicene and Post-Nicene Fathers, Ser. II, Vol. XI:
The Works of John Cassian.: Chapter III. His answer to the question raised.

Chapter III.

His answer to the question raised.

A threefold account of this mental dryness of which you speak has been given by the Elders. For it comes either from carelessness on our part, or from the assaults of the devil, or from the permission and allowance of the Lord. From carelessness on our part, when through our own faults, coldness has come upon us, and we have behaved carelessly and hastily, and owing to slothful idleness have fed on bad thoughts, and so make the ground of our heart bring forth thorns and thistles; which spring up in it, and consequently make us sterile, and powerless as regards all spiritual fruit and meditation. From the assaults of the devil when, sometimes, while we are actually intent on good desires, our enemy with crafty subtilty makes his way into our heart, and without our knowledge and against our will we are drawn away from the best intentions.