The problem of the tides may be illustrated by a parable. Once there was a keen, unimaginative observer living on a seacoast, where a perpetual pall of clouds covered the sky, concealing the sun and moon, but where the tides, with their periodic variations, were familiar matters; he would gain a good knowledge of the facts of observation, but he would have no knowledge of their meaning as revealed by the unseen facts of inference. At the same time a philosophical hermit was living alone under the clear skies of a desert continental interior, where he was totally ignorant of the oceans and their tides, but familiar with the motions of the sun and moon, and acquainted with the law of gravitation, in accordance with which the heavenly bodies move; he might from this beginning go on with a series of inferences, or deductions, which would in the end lead him to say: “These distant bodies must exert unequal attractions on different parts of the earth, but the earth is too rigid to yield to them; if, however, a large part of the earth’s surface were covered with a sheet of water, the attractions of the sun and moon would produce periodic variations in the level of such a sheet”. . . and so on. After a time, the long-shore observer sets out upon his travels and meets the hermit in the interior desert, who asks him: “Do you happen to have seen a large sheet of water, in which periodic changes of level take place?” “I have indeed,” the observer exclaims, “and I was on the point of telling you about the changes of level in the hope that you could explain them; but how did you know that the changes occurred?” “I did not even know,” the hermit replies, “that there was a vast sheet of water in which they could occur; but I felt sure that, if such a water sheet existed, it must suffer periodic changes of level, because ...” The evident point of the parable is that the keen observer and the speculative hermit are both combined in a trained scientific investigator; he performs the two tasks of observation and of explanation independently, as if he were two persons; and his philosophical half finally accepts as true that particular scheme or theory which leads to the best understanding of the facts gained by his observational half.

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