“6. Greek mathematicians did not think one was a number because the concept one did not involve number. To them, two was the first number. And the hybrid marriage of one, which was not a number, and two, which was, begot three, the second number. And from one, two, and three, all other numbers proceeded, so that all odd numbers had in them an element that was not number. This is why Plato said that the leap from one to two was the leap to rationality.
Leonard Bernstein, speaking of music, said that two was a rhythm of the body and three was a rhythm of the mind. This has been contested by people who say that three is a rhythm of the body and two a rhythm of the mind. Not everyone has weighed in on this subject. But it seems intuitively right, doesn’t it? To say that there is a groundedness in the symmetry of twos, off which threes seem to play, seem airier.”
Robert Hass, from “A Little Book on Form”