Such watchfulness blocks demonic thoughts from invading the heart, enabling the mind to concentrate on ‘the one thing needful’… ‘The mind of an attentive man is the sentry, the sleepless guardian, placed over the inner Jerusalem.’
This passage establishes watchfulness (nepsis) as the central spiritual-psychological practice of the Philokalia tradition, explicitly linking vigilance over the heart to the exclusion of invasive ideation and the achievement of theosis.
, Philokalia: The Bible of Orthodox Spirituality, 1998thesis