"You ask if I believe that spirits come back, and if the circumstance which occurred at St. Maur can be attributed to one of those incorporeal substances?
"To answer your two questions in the same order that you propose them to me, I must first tell you, that the ancient heathens acknowledge various kinds of spirits, which they called lares, larvæ, lemures, genii, manes.
"For ourselves, without pausing at the folly of our cabalistic philosophers, who fancy spirits in every element, calling those sylphs which they pretend to inhabit the air; gnomes, those which they feign to be under the earth; ondines, those which dwell in the water; and salamanders, those of fire; we acknowledge but three sorts of created spirits, namely, angels, demons, and the souls which God has united to our bodies, and which are separated from them by death.
"The Holy Scriptures speak in too many places of the apparitions of the angels to Abraham, Jacob, Tobit, and several other holy patriarchs and prophets, for us to doubt of it. Besides, as their name signifies their ministry, being created by God to be his messengers, and to execute his commands, it is easy to believe that they have often appeared visibly to men, to announce to them the will of the Almighty. Almost all the theologians agree that the angels appear in the aërial bodies with which they clothe themselves.
"To make you understand in what manner they take and invest themselves with these bodies, in order to render themselves visible to men, and to make themselves heard by them, we must first of all explain what is vision, which is only the bringing of the species within the compass of the organ of sight. This "species" is the ray of light broken and modified upon a body, on which, forming different angles, this light is converted into colors. For an angle of a certain kind makes red, another green, blue or yellow, and so on of all the colors, as we perceive in the prism, on which the reflected rays of the sun forms the different colors of the rainbow; the species visible is then nothing else than the ray of light which returns from the object on which it breaks to the eyes.
"Now, light falls only on three kinds of objects or bodies, of which some are diaphanous, others opake, and the others participate in these two qualities, being partly diaphanous and partly opake. When the light falls on a diaphanous body which is full of an infinity of little pores, as the air, it passes through without causing any reflection. When the light falls on a body entirely opake, as a flower, for instance, not being able to penetrate it, its ray is reflected from it, and returns from the flower to the eye, to which it carries the species, and renders the colors distinguishable, according to the angles formed by reflection. If the body on which the light falls is in part opake and in part diaphanous, like glass, it passes through the diaphanous part, that is to say, through the pores of the glass which it penetrates, and reflects itself on the opake particles, that is to say, which are not porous. Thus the air is invisible, because it is absolutely penetrated with light: the flower sends back a color to the eye, because, being impenetrable to the light, it obliges it to reflect itself; and the glass is visible only because it contains some opake particles, which, according to the diversity of angles formed upon it by the ray of light, reflect different colors.
"That is the manner in which vision is formed, so that air being invisible, on account of its extreme transparency, an angel could not clothe himself with it and render himself visible, but by thickening the air so much, that from diaphanous it became opake, and capable of reflecting the ray of light to the eye of him who perceived him. Now, as the angels possess knowledge and power far beyond anything we can imagine, we need not be astonished if they can form aërial bodies, which are rendered visible by the opacity they impart to them. In respect to the organs necessary to these aërial bodies, to form sounds and make themselves heard, without having any recourse to the disposition of matter, we must attribute them entirely to a miracle.
"It is thus that angels have appeared to the holy patriarchs. It is thus that the glorious souls that participate the angelic nature can assume an aërial body to render themselves visible, and that even demons, by thickening and condensing the air, can make to themselves a body of it, so as to become visible to men, by the particular permission of God, to accomplish the secrets of his providence, as they are said to have appeared to St. Anthony the Hermit, and to other saints, in order to tempt them.
"Excuse, sir, this little physical digression, with which I could not dispense, in order to make you understand the manner in which angels, who are purely spiritual substances, can be perceived by our fleshly senses.
"The only point on which the holy doctors do not agree on this subject is, to know if angels appear to men of their own accord, or whether they can do it only by an express command from God. It seems to me that nothing can better contribute to the decision of this difficulty, than to determine the way in which the angels know all things here below; for if it is by means of "species" which God communicates to them every day, as St. Augustine believes, there is no reason to doubt of their knowing all the wants of mankind, or that they can, in order to console and strengthen them, render their presence sensible to them, by God's permission, without receiving an express command from him on the subject; which may be concluded from what St. Ambrose says on the subject of the apparition of angels, who are by nature invisible to us, and whom their will renders visible. Hujus naturæ est non videri, voluntatis, videri.[[662]]