A great many things have been pronounced untrue and absurd, and even impossible, by the highest authorities of the age in which they lived, which have afterward, and indeed within a very short period, been found to be both possible and true. I confess myself, for one, to have no respect whatever for these dogmatic denials and affirmations, and I am quite of opinion that vulgar incredulity is a much more contemptible thing than vulgar credulity. We know very little of what _is_, and still less of what may be; and till a thing has been proved, by induction, logically impossible, we have no right whatever to pronounce that it is so. As I have said before, _a priori_ conclusions are perfectly worthless; and the sort of investigation that is bestowed upon subjects of the class of which I am treating, something worse—inasmuch as they deceive the timid and the ignorant, and that very numerous class which pins its faith on authority and never ventures to think for itself, by an assumption of wisdom and knowledge, which, if examined and analyzed, would very frequently prove to be nothing more respectable than obstinate prejudice and rash assertion. For my own part, I repeat, I insist upon nothing. The opinions I have formed, from the evidence collected, may be quite erroneous; if so, as I seek only the truth, I shall be glad to be undeceived, and shall be quite ready to accept a better explanation of these facts, whenever it is offered to me: but it is in vain to tell me that this explanation is to be found in what is called imagination, or in a morbid state of the nerves, or an unusual excitement of the organs of color and form, or in imposture; or in all these together. The existence of all such sources of error and delusion I am far from denying, but I find instances that it is quite impossible to reduce under any one of those categories, as we at present understand them. The multiplicity of these instances, too—for, not to mention the large number that are never made known or carefully concealed, if I were to avail myself liberally of cases already recorded in various works, many of which I know, and many others I hear of as existing, but which I can not conveniently get access to, I might fill volumes (German literature abounds in them)—the number of the examples, I repeat, even on the supposition that they are not facts, would of itself form the subject of a very curious physiological or psychological inquiry. If so many people in respectable situations of life, and in apparently a normal state of health, are capable of either such gross impostures, or the subjects of such extraordinary spectral illusions, it would certainly be extremely satisfactory to learn something of the conditions that induce these phenomena in such abundance; and all I expect from my book at present is, to induce a suspicion that we are not quite so wise as we think ourselves; and that it might be worth while to inquire a little seriously into reports,