Guarding of the Heart

“Our holy fathers hearkened to the Lord’s words, ‘Out of the heart proceed evil thoughts, murders, adulteries, unchastity, thefts, perjuries, blasphemies; these are the things that defile a man’ (Mt. 15:19-20); and they also hearkened to Him when He enjoins us to cleanse the inside of the cup so that the outside may also be clean (cf. Mt. 23:26). Hence they abandoned all other forms of spiritual labor and concentrated wholly on this one task of guarding the heart, convinced that through this practice they would also possess every other virtue, whereas without it no virtue could be firmly established. Some of the fathers have called this practice stillness of the heart, others attentiveness, others the guarding of the heart, others watchfulness and rebuttal, and others again the investigation of the thoughts and the guarding of the intellect. But all of them alike worked the earth of their own heart, and in this way they were fed on the divine manna (cf. Ex. 16:15).”
~St. Symeon the New Theologian