The Philokalia has one subject, and it is not asceticism in the popular sense of fasting and vigils. Its subject is nepsis, watchfulness, the sober attention a person pays to the movements of their own mind, and the discrimination by which a thought is noticed as it arrives, before it has become consent. The vocabulary is consistent across eleven centuries of authors: logismoi, the thoughts that present themselves; prosoche, attentiveness; diakrisis, discernment; apatheia, the state in which the passions no longer determine what one does; nous, the faculty of direct spiritual perception, which the tradition locates in the heart rather than the head and which is not the same thing as the intellect.
Bound up with watchfulness is unceasing prayer, and above all the Jesus Prayer: "Lord Jesus Christ, Son of God, have mercy on me, a sinner", repeated until it continues on its own. The later texts describe a physical method (a posture, attention to the breath, the mind brought down into the heart), which was controversial in its own time and drew the charge from Barlaam of Calabria that the hesychast monks were omphalopsychoi, navel-gazers. Gregory Palamas defended it, and the councils of the 1340s and 1350s vindicated him; his distinction between God's unknowable essence and his uncreated energies, in which the divine light of Tabor is genuinely God and genuinely perceptible, became Orthodox doctrine and is the theological backbone of the collection's last layer.