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.