…if you are rightly cultivating stillness and aspiring to be with God, and you see something either sensory or noetic, within or without, be it even an image of Christ or of an angel or of some saint, or you imagine you see a light in your intellect and give it a specific form, you should never entertain it. For the intellect itself naturally possesses an imaginative power and in those who do not keep a strict watch over it it can easily produce, to its own hurt, whatever forms and images it wants to. In this way the recollection of things good or evil can suddenly imprint images on the intellect’s perceptive faculty and so induce it to entertain fantasies, thus making whoever this happens to a daydreamer rather than a hesychast.
~St Gregory of Sinai (Philokalia, vol.4, p.283)
Prayerful Life 