A man acquainted with the history of the sciences, and who reasons calmly, cannot succeed in understanding the ostracism to which certain sceptics subject unexplained phenomena. "It is impossible," they think. This famous common sense on which they plume themselves is nothing after all, let me say, but common opinion, which accepts habitual facts without comprehending them, and which varies from time to time. What man of good sense would formerly have admitted that we should one day be able to photograph the skeleton of a living being, or store up the voice in a phonograph, or determine the chemical composition of an inaccessible star? What was science a hundred years ago, two hundred years, three hundred? Look at astronomy five hundred years ago, and physiology, and medicine, and natural philosophy, and chemistry. In five hundred years, in a thousand years, in two thousand years, what will these sciences of ours be? And in a hundred thousand years? Yes, in a hundred thousand years, what will human intelligence be? Our actual condition will be to that what the knowledge of a dog is to that of a cultivated man; that is to say, there is no possible comparison. We smile to-day at the science of learned men of the time of Copernicus or Christopher Columbus or Ambroise Pare, and we forget that, in a few centuries, savants will estimate us in the same fashion. There are properties of matter which are completely hidden from us, and humanity is endowed with faculties still unknown to us. We only advance very slowly in the knowledge of things. The critics do not always give proof that they possess a very compact logical power. You speak to them of facts proved by centuries of testimony. They challenge the value of popular testimony, and declare that these uncultivated folks, these petty merchants, these manufacturers, these laborers, these peasants, are incapable of observing with any exactitude. Some days after, you cite the savants, men whose competence has been proved in the objective sciences of observation, which attest these very facts, and you hear the sneerers answer that those savants are competent witnesses in their special lines of study and work, but in nothing apart from these. So, after this fashion, all testimony is refused. They declare that the thing, being impossible, cannot have been observed at all. Of course there is room for a good deal of analysis in discussing the claims of human testimony. But, if we suppress every piece of testimony, what will there be left?--our native ignorance. But, to tell the truth, there are some of these negative gentry who are sure of everything, and who impose their aphorisms upon us with the authority of a czar giving out his ukase or edict.