A sound which is stable and which does not have the urgency to resolve is called a consonance. This term refers to chords as well as to intervals. The consonant intervals are: all perfect intervals (except the perfect fourth in certain instances) and the major and minor thirds and sixths. They are divided into two groups, the perfect and imperfect consonances, their placement being determined by their degree of stability.
If it be asked why no more primes than 2, 3, and 5 are admitted into musical ratios, one reason is that consonances whose vibrations are in ratios whose terms involve 7, 11, 13, etc., would be less simple and harmonious than those whose terms involve the lesser primes only. Another reason is this - as perfect fifths and other intervals resulting from the number 3 make the schism of a comma with perfect thirds and other intervals resulting from the number 5, so intervals resulting from the numbers 7, 11, 13, etc., would make other schisms with both those kinds of intervals. [Scientific Basis and Build of Music, page 75]