 The most notable opponent of the Pythagorean notion of celestial harmony was Aristotle. He offers two main reasons to reject this idea. First, he argues that physical bodies could not be composed of numbers, for the latter have no weight, whereas the former do (Metaphysics, 14.3). Second, he observes that loud sounds shatter glass and produce a physical effect upon us. The movement of planets in space, Aristotle argues, would produce an even greater effect on us, the absence of which is proof that celestial bodies produce no sound (On the Heavens, 2.9). In addition to rejecting the idea of a celestial harmony, Aristotle also rejects the related notion that the human soul is a kind of harmony (On the Soul, 1.4).