Prayer and fasting banish all sinful thoughts, stop the fever of the brain, preserve the mind from
disturbance, gather it away from dispersion, and enlighten it. Prayer and fasting banish from a man unclean spirits and heal those who are tormented by demons, as the Lord said:
This kind (that is, the devils) goeth not out save by prayer and fasting (Matt. 17:21). Prayer and fasting also cleanse the mind in which the devils often cause darkness. Prayer and fasting are
weapons which protect every access of the devils to us.
Prayer, like a grace-given spirit, enlightens the mind, and gives fervor. Prayer with sobriety of
heart is the guardian of the mind, extinguishes its forgetfulness as water does fire. Therefore, one should call by an honorably name this virtue which gives birth to bright visions.
St. Paisius Velichkovsky