Harmonia tou kosmou — the doctrine that the celestial bodies produce a music through their orbital motion — originates with Pythagoras, who, according to Porphyry (Life of Pythagoras), was the only mortal able to hear the celestial sounds. The foundation is the discovery that consonant musical intervals arise from simple arithmetical ratios (the octave at 2:1, the fifth at 3:2, the fourth at 4:3), extended by analogy to the distances and velocities of the planetary spheres. Aristotle discusses and critiques this doctrine in De Caelo (290b). For the Pythagoreans, harmonia was not metaphor but ontology: the kosmos itself is an ordered arrangement sustained by proportional relationships, and its inaudibility to mortals results from our continuous immersion in its sound since birth.