23. Holy Scripture speaks everywhere about the guarding of the heart, in both the Old and the New Testaments. David says in the Psalms: ‘O sons of men, how long will you be heavy of heart?’ (Ps. 4:2. LXX), and again: ‘Their heart is vain’ (Ps. 5:9. LXX); and of those who think futile thoughts, he says: ‘For he has said in his heart, I shall not be moved’ (Ps. 10:6), and: ‘He has said in his heart, God has forgotten’ (Ps. 10:11).
A monk should consider the purpose of each text in Scripture, to whom it speaks and on what occasions. He should persevere continually in the ascetic struggle and be on his guard against the provocations of the enemy. Like a pilot steering a boat through the waves, he should hold to his course, guided by grace. Keeping his attention fixed within himself, he should commune with God in stillness, guarding his thoughts from distraction and his intellect from curiosity.
(Philokalia V1 P. 27, St Isaiah the Solitary, On Guarding the Intellect)