Lights And Graces Of Sincere Sanctity

In supernatural imaginative vision an agent superior to man acts directly either on the imagination itself or on certain forces calculated to stir the imagination. The sign that these images come from God lies, apart from their particular vividness, in the lights and graces of sincere sanctity which accompany them, and in the fact that the subject is powerless to define or fix the elements of the vision. Such efforts most frequently result in the cessation or the abridgement of the vision. 

Imaginative apparitions are ordinarily of short duration, either because the human organism is unable to endure for a long time the violence done to it, or imaginative visions soon give place to intellectual visions. This kind of vision occurs most frequently during sleep; 

such were the dreams of Pharaoh and Nebuchadnezzar (Genesis 41; Daniel 2). Cardinal Bona gives several reasons of expediency for this frequency: during sleep the soul is less divided by multiplicity of thoughts, it is more passive, more inclined to accept, and less inclined to dispute; in the silence of the senses the images make a more vivid impression.

It is often difficult to decide whether the vision is corporeal or imaginative. It is certainly corporeal (or extrinsic) if it produces external effects, such as the burnt marks left on an object by the passing of the Devil. It is imaginative if, for example, the image persists after one has closed one's eyes, or if there are no traces of the external effects which ought to have been produced, such as when a ball of fire appears above a person's head without injuring it. 

The time most conducive to these visions is a state of ecstasy, when the exercise of the external senses is suspended. However, although the question has been discussed among mystics, it seems that they may also be produced outside of this state. This is the opinion of  the Jesuit Alvarez de Paz (here) , (here) , (here) (De grad. contemp., 1., V, pt. III, cii, t. 6) and of Benedict XIV (De servorum Dei beatif., 1. III, c. i, n. 1). 

Imaginative vision may be either representative or symbolic. It is representative when it presents an image of the very object to be made known: such may have been the apparition to St. Joan of Arc of St. Catherine and St. Margaret, if it was not (which is more probable) a luminous vision. It is symbolic when it indicates the object by means of a sign: such as the apparition of a ladder to Jacob, the apparition of the Sun, Moon, and stars to the patriarch Joseph, as were also numerous prophetic visions.

Link (here) to the full article Visions and Apparitions found in the Catholic Encyclopedia, copyright © 1913 by the Encyclopedia Press, Inc.