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 a magnet—I suppose you have iron and loadstones here as we have on Earth—was, to the first man who witnessed it, just as complete a violation of the law of gravity as now appears my voyage through space, accomplished by a force bearing some relation to that which acts through the magnet."

"Our philosophers," he answered, "are probably satisfied that they know nearly all that is to be known of natural laws and forces; and to delusion or illusion human sense is undeniably liable."

"If," I said, "you cannot trust your senses, you may as well disbelieve in your own existence and in everything around you, for you know nothing save through those senses which are liable to illusion. But we know practically that there are limits to illusion. At any rate, your maxim leads directly and practically to the inference that, since I do not belong to Mars and cannot have come from any other world, I am not here, and in fact do not exist. Surely it was somewhat illogical to shoot an illusion and intend to dissect a spectre! Is not a fact the complete and unanswerable refutation of its impossibility?"

"A good many facts to which I could testify," he replied, "are in this world confessed impossibilities, and if my neighbours witnessed them they would pronounce them to be either impostures or illusions."

"Then," said I, somewhat indignantly, "they must prefer inferences from facts to facts themselves, and the deductions of logic to the evidence of their senses. Yet, if that evidence be wanting in certainty, then, since no chain can be stronger than its weakest point, inferences are doubly uncertain; first, because they are