St Diadochos of Photiki. "On Spiritual Knowledge and Discrimination. One Hundred Texts". From Philokalia, Vol. 1.
36. Let no one who hears us speak of the perceptive faculty of the intellect imagine that by this we mean that the glory of God appears to man visibly. We do indeed affirm that the soul, when pure, perceives God's grace, tasting it in some ineffable manner; but no invisible reality appears to it in a visible form, since now 'we walk by faith, not by sight', as St Paul says (2 Cor. 5:7) light or some fiery form should be seen by one pursuing the spiritual way, he should not on any account accept such a vision: it is an obvious deceit of the enemy. Many indeed have had this experience and, in their ignorance, have turned aside from the way of truth. We ourselves know, however, that so long as we dwell in this corruptible body, 'we are absent from the Lord' (2 Cor. 5:6) - that is to say, we know that we cannot see visibly either God Himself or any of His celestial wonders.
37. The dreams which appear to the soul through God's love are unerring criteria of its health. Such dreams do not change from one shape to another; they do not shock our inward sense, resound with laughter or suddenly become threatening. But with great gentleness they approach the soul and fill it with spiritual gladness. As a result, even after the body has woken up, the soul longs to recapture the joy given to it by the dream. Demonic fantasies, however, are just the opposite: they do not keep the same shape or maintain a constant form for long. For what the demons do not possess as their chosen mode of life, but merely assume because of their inherent deceitfulness, is not able to satisfy them for very long. They shout and menace, often transforming themselves into soldiers and sometimes deafening the soul with their cries. But the intellect, when pure, recognizes them for what they are and awakes the body from its dreams. Sometimes it even feels joy at having been able to see through their tricks; indeed it often challenges them during the dream itself and thus provokes them to great anger. There are, however, times when even good dreams do not bring joy to the soul, but produce in it a sweet sadness and tears unaccompanied by grief. But this happens only to those who are far advanced in humility.
38. We have now explained the distinction between good and bad dreams, as we ourselves heard it from those with experience. In our quest for purity, however, the safest rule is never to trust to anything that appears to us in our dreams. For dreams are generally nothing more than images reflecting our wandering thoughts, or else they are the mockery of demons. And if ever God in His goodness were to send us some vision and we were to refuse it, our beloved Lord Jesus would not be angry with us, for He would know we were acting in this way because of the tricks of the demons. Although the distinction between types of dreams established above is precise, it sometimes happens that when the soul has been sullied by an unperceived beguilement - something from which no one, it seems to me, is exempt - it loses its sense of accurate discrimination and mistakes bad dreams for good.