Nicene and Post-Nicene Fathers, Ser. II, Vol. XI:
The Works of John Cassian.: Chapter XVII. The answer what the mind can and what it cannot do with regard to the state of its thoughts.

Chapter XVII.

The answer what the mind can and what it cannot do with regard to the state of its thoughts.

Moses. It is impossible for the mind not to be approached by thoughts, but it is in the power of every earnest man either to admit them or to reject them. As then their rising up does not entirely depend on ourselves, so the rejection or admission of them lies in our own power. But because we said that it is impossible for the mind not to be approached by thoughts, you must not lay everything to the charge of the assault, or to those spirits who strive to instil them into us, else there would not remain any free will in man, nor would efforts for our improvement be in our power: but it is, I say, to a great extent in our power to improve the character of our thoughts and to let either holy and spiritual thoughts or earthly ones grow up in our hearts. For for this purpose frequent reading and continual meditation on the Scriptures is employed that from thence an opportunity for spiritual recollection may be given to us, therefore the frequent singing of Psalms is used, that thence constant feelings of compunction may be provided, and earnest vigils and fasts and prayers, that the mind may be brought low and not mind earthly things, but contemplate things celestial, for if these things are dropped and carelessness creeps on us, the mind being hardened with the foulness of sin is sure to incline in a carnal direction and fall away.