The Pythagoreans' discovery that there was a relationship between musical intervals and numerical ratios led to the belief that the study of mathematics was the key to the understanding of the structure and order of the universe. Astronomy and harmony, they said, were sister sciences, one for the eyes and one for the ears). However, it was not until two millennia later that Galileo and his successors showed the sense in which it is true that the book of the universe is written in numbers.