So we can speak of the blindness and inability of the nous to see things clearly. And when our nous is darkened, we do not have a pure and open passage to our neighbor. Everything is defiled and darkened, with terrible and upsetting consequences for our life. Just as clouds hide the sun, so evil thoughts bring shadows to the mind and ruin it. Our nous is darkened and remains unproductive either when we speak words of worldly import or, entertaining such words in our mind, we associate with them, or when our body involves itself with the nous in sensory things. Then we immediately lose our fervor, compunction, intimacy with God and spiritual knowledge. Therefore “so long as we concentrate our attention on the nous, we are enlightened; but when we are not attentive to it we are in darkness” [Hesychios the Priest, The Philokalia, Vol. 1, p. 184].
Metropolitan of Nafpaktos Hierotheos, Orthodox Psychotherapy, p. 135.