"Watchfulness cleanses the conscience and makes it lucid. This cleansed, it immediately shines out like a light that has been uncovered, banishing much dartkness, … He who has tasted this light will understand what I am talking about. The soul is never sated with it, but the more it feeds on it, the more hungry it grows. It is a light that attracts the intellect as the sun the eyes. Inexplicable, it yet becomes explicable through experience. This experience I have known or, more precisely, I have been wounded by it; but it commands me to be silent, even though my intellect would delight in speaking of it."
St. Philotheos of Sinai, Forty Texts on Watchfulness, #24,(roughly eighth or ninth century), The Philokalia, Vol. III., page 25. (via theuniverseworks)