In the time of contemplation we must keep our intellect free of all fantasy and image …
(St. Diadochos of Photiki)
The effect of observing the commandments is to free from passion our conceptual images of things. The effect of spiritual reading and contemplation is to detach the intellect from form and matter. It is this which gives rise to undistracted prayer.
(St. Maximos the Confessor)
The fifth form of discipline consists in spiritual prayer, prayer that is offered by the intellect and free from all thoughts. During such prayer the intellect is concentrated within the words spoken and, inexpressibly contrite, it abases itself before God, asking only that His will may be done in all its pursuits and conceptions. It does not pay attention to any thought, shape, color, light, fire or anything at all of this kind; but, conscious that it is watched by God and communing with Him alone, it is free from form, color and shape.
(St. Peter of Damaskos)