Here is a dilemma, which appears to me to be very conclusive in that respect:—Either we know all Nature’s forces, or we do not. Now the first alternative is so ridiculous, that it is really not worth while refuting it. Our senses are so limited, so imperfect, that the world slips away from them almost entirely. We may say it is owing to an accident, that the magnet’s colossal force was discovered, and if hazard had not placed iron beside the loadstone, we might have always remained ignorant of the attraction which loadstone exercises upon iron. Ten years ago no one suspected the existence of the Rœntgen rays. Before photography, no one knew that light reduces salts of silver. It is not twenty years since the Hertzian waves were discovered. The property displayed by amber when rubbed was, until two hundred years ago, all that was known of that immense force called electricity. Question a savage—nay a fellah or a moujik—upon the forces of Nature! He will not know even the tenth part of such forces as elementary treatises on physics in 1905 will enumerate. It appears to me that the savants of to-day, in respect to the savants of the future, stand in the same inferiority as the moujiks to the professors of the college of France. Who then dare be so rash as to say that the treatises on physics in 2005 will but repeat what is to be found in the treatises of 1905? The probability—the certainty, one might say—is that new scientific data will shortly spring up out of the darkness, and that most powerful and altogether unknown forces will be revealed. Our great-grandchildren will be amazed at the blindness of our savants, who tacitly profess the immobility of science. If science has made such progress of late, it is precisely because our predecessors were not afraid to make bold hypotheses, to suppose new forces, demonstrating their reality by dint of patience and perseverance. Our strict duty is to do likewise. The savant should be a revolutionist, and fortunately the time is over when truth had to be sought in a master’s book—_magister dixit_—be he Aristotle or Plato. In politics we may be conservative or progressive; it is a question of temperament. But when the research of truth is concerned we must be resolutely and unreservedly revolutionary, and must consider classical theories—even those which appear to be the most solid—as temporary hypotheses, which we must incessantly check and incessantly strive to overthrow. The Chinese believed that science had been fixed by their ancestors’ sapience; this example contains food for meditation.