"Mr. Dalton's permanent reputation will rest upon his having discovered a simple principle, universally applicable to the facts of chemistry - in fixing the proportions in which bodies combine, and thus laying the foundation for future labors... his merits in this respect resemble those of Kepler in astronomy."

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Laplace considers astronomy a science of observation, because we can only observe the movements of the planets; we cannot reach them, indeed, to alter their course and to experiment with them. "On earth," said Laplace, "we make phenomena vary by experiments; in the sky, we carefully define all the phenomena presented to us by celestial motion." Certain physicians call medicine a science of observations, because they wrongly think that experimentation is inapplicable to it.

— Claude Bernard