De institutione musica I.32: Quae consonantia quam merito praecedat.
But among all the consonances we have mentioned, a judgment must be had, in reason as in the ear, as to which of them ought to be thought better. For the ear is affected by sounds, or the eyes by sight, in the same way as the judgment of the mind by numbers or by continuous quantity. For given a number or a line, nothing is easier than to judge its double, which is the diapason, its triple, and the rest according to the same manner and form. But Ptolemy does not judge this in the same way, and his whole opinion I shall set out later.