“I tell you I saw it myself,” is the so-thought conclusive assertion with which many a controversy is abruptly ended. Commonly those who make this assertion think that after it nothing remains to be urged; and they are astonished at the unreasonableness of those who still withhold their belief. Though they reject many tales of witchcraft, many ghost stories whose marvels were attested by eye-witnesses—though they have repeatedly seen stage-conjurors seem to do things which they do not believe were really done—though they have heard of the Automaton Chess-player and the Invisible Girl, and have perhaps seen explanations of the modes in which the public were deluded by {162} them—though in all these cases they know that the facts were other than the spectators supposed them to be; yet they cannot imagine that their own perceptions have been vitiated by influences like those which vitiated the perceptions of others. Or, to put the thing more charitably and perhaps more truly, they forget that such vitiations are constantly occurring. To observe correctly, though commonly thought very easy, every man of science knows to be difficult. Our faculties are liable to report falsely from two opposite causes—the presence of hypothesis, and the absence of hypothesis. To the dangers arising from one or other of these, every observation we make is exposed; and between the two it is hard to see any fact _quite_ truly. A few illustrations of the extreme distortions arising from the one cause, and the extreme inaccuracy consequent on the other, will justify this seeming paradox.