"Hear thou, my son, and be wise, and guide thine heart in the way (Prov. 23:19). Out of the heart continually proceed thoughts which sometimes are good, but more often are evil. The evil ones should not be followed at all, but even the good ones should not always be carried out. It happens that even thoughts which are good in and of themselves are inappropriate in reality, due to circumstances. This is why it is prescribed to be attentive toward oneself, to keep an eye on all that proceeds out of the heart—to reject the evil, consider what is good, and fulfil only what proves to be truly good. But best of all would be to totally imprison the heart, so that nothing leaves it and nothing enters it without the permission of the mind; so that the mind would come first in all things, determining the movements of the heart. But the mind is this way only when it is the mind of Christ. Thus, unite with Christ in mind and heart and everything within you will be in good working order."
- Saint Theophan the Recluse