“Marquis, Marquis!” the Duke said, smiling, “you use the ghost-seers very ill. I should leave them entirely at your mercy, if the Irishman had not promised to communicate to me a criterion by which one can discern real apparitions from vain phantoms of the imagination.”
“It is a pity he has only promised it, it being probable that this promise will not be performed with greater punctuality than the rest of his engagements.”
“The event will prove how much you wrong him.”
“But what would you say, if I could prove that he can communicate to you no criterion of that nature?”
“If you could do this---”
“Nothing is easier. The criterion whereby a real apparition of a ghost could be discerned from an illusion, must be either external or internal: that is, you must be able to ascertain the presence of a ghost, either by means of your senses, or by conclusions deduced from the impression your mind receives. Don’t you think so?”
“It would be much safer if these two criterions co-existed.”
“It would be sufficient if only one of these two criterions were possible. However, you shall soon be convinced that neither can be proved. Whatever you perceive, or suppose you perceive by means of your senses, in case of an apparition, is either a real material object, whereby perhaps an impostor, perhaps nature, who is so inexhaustable in her effects, or an accidental meeting of uncommon incidents surprises you; or it is an object that exists no where but in your heated imagination; what you perceive through your senses never can be the spirit himself, because spirits are incorporeal beings, and therefore neither can be seen, heard, nor felt; it is, consequently, evident that no external criterion of the reality of an apparition can exist.”
“This, I think, cannot be disputed.”
“But there exists perhaps an internal criterion. In order to decide this question, let us consider what passes in the human mind when a ghost appears. First of all, a lively idea of the presence of a ghost takes place, and sensations of terror, astonishment and awe arise---however, this idea and these sensations may be nothing else but the consequence of an uncommon, though natural external impression of a feverish fancy, and consequently never can be indubitable proofs of the presence of spirits. But perhaps the presence of spirits is ascertained by the co-existence of certain extraordinary notions, sensations, and cognitions! This too cannot be, for we must be convinced that they could not arise in our soul in a natural manner, if we shall be able to ascertain their having been produced by the influence of a spirit. In that case it would be requisite we should know the whole store of our clear and obscure ideas, all their reciprocal relations, and all possible compositions which our imagination can form of them, a knowledge that is reserved only for the omniscient Ruler of the world. If we happen sometimes, in our dreams, to have the most wonderful visions, to reason in the most sensible manner, to discover new truths, and to predict incidents which afterwards really happen; why should not the same faculty of the soul which produces such uncommon effects in our dreams, surprise us sometimes with similar operations while we are awake, when it is agitated in a violent manner? In short, my friend, there exists neither an internal nor an external criterion whereby we could ascertain the reality of an apparition.”