to all which concerns the "supernatural" (using that term in its generally understood sense, without attempting a logical definition of it), the allowance that has to be made for "prepossession" is so large as practically to destroy the validity of any testimony which is not submitted to the severest scrutiny according to the strictest scientific methods. Of the manner in which, within my own experience, what seemed the most trustworthy testimony has been completely discredited by the application of such methods, I shall give some examples hereafter.
I would by no means claim for myself or any other scientific man an immunity from idolatrous prepossessions; for we must all be guided in our researches by some notion of what we expect to find; and this notion may be very misleading. Thus, when no metal was known that is not several times heavier than water, it was not surprising that Dr. Pearson, as he poised upon his finger the first globule of potassium produced by the battery of Davy, should have exclaimed, "Bless me, how heavy it is!" though, when thrown into water, the metal floated upon it. But while the true disciple of Bacon is on his guard against "idolatry," and is constantly finding himself rudely handled (as Dr. Pearson was) by "the irresistible logic of facts" if he falls into it, the pledged upholder of any religious system can be scarcely other than, in some degree, an "idolater." The real philosopher, says Schiller, is distinguished from the "trader in knowledge" by his "always loving truth better than his system."
Bacon's classification of "idols" is based on the sources of our prepossessions; and, although his four types graduate insensibly into each other, yet the study of them is very profitable. Sir John Herschel is, I think, less successful when he classifies them as—1. Prejudices of opinion, and 2. Prejudices of sense; because an analysis of any of his "prejudices of sense" shows that it is really a "prejudice of opinion." My first object is to show that we are liable to be affected by our prepossessions at every stage of our mental activity, from our primary reception of impressions from without, to the highest exercise of our reasoning powers; and that the value of the testimony of any individual, therefore, as to any fact whatever, essentially depends upon his freedom from any prepossessions that can affect it.
That our own states of consciousness constitute what are, to each individual, the most certain of all truths—in a philosophical sense (as J. S. Mill says) the only certain truths—will, I suppose, be generally admitted; but there is a wide hiatus between this and the position that every state of consciousness which represents an external object has a real object answering to it. In fact, although we are acccustomed to speak of "the evidence of our senses" as worthy of the highest credit, nothing is easier than to show that the evidence of any one sense, without the check afforded by comparison with that of another, is utterly untrustworthy.
