Our holy fathers have listened to the Lord who said that from the heart come evil intentions, murder, adultery, fornication, theft, perjury, slander [Mt 15:19] and how these are the things that make a man unclean. [Mt 15:20] Further, they have listened to the part of the gospel where we are ordered to clean the inside of cup and dish first so that the outside may become clean as well. [Mt 23:26] They therefore left aside any other spiritual work and concentrated exclusively on guarding the heart, being confident that through this they would easily achieve all other virtues, whilst without it no virtue can be preserved. This practice was called by some fathers ‘serenity of the heart,’ whilst others named it ‘attention,’ others ‘sobriety’ and ‘detainment,’ others ‘examination of the thoughts’ and ‘guarding of the mind;’ for they were all absorbed in this, and by this they were found worthy to accept the divine virtues.
Based on St. Simeon the New Theologian