Probably more nonsense has been talked about dissonance than anything else in music. To clear away this inherited confusion, let us start with a few simple principles.
First, a point of terminology. Like heat and cold or light and dark, consonance and dissonance are opposite ways of measuring the same property. Just as, to the physicist, cold is merely the absence of heat, and dark the absence of light, so dissonance is merely the absence of consonance. In strict logic, this chapter might equally well have been entitled 'Consonance and Concord', but there are two good practical reasons for using the negative terms. One is that total consonance and concord are rare, being in fact confined to the unison and octave;1 and the other is that dissonance and discord, from the point of view of the artist (as opposed to the scientist), are positive qualities. They are what gives music its interest and vitality.
Secondly, the words 'dissonance' and 'discord' (at least as used in this book) stand for two quite different things. Dissonance is a property of successive notes; discord, of simultaneous notes.
Thirdly, these two properties are apprehended in very different ways. Dissonance, like the melody of which it forms part, belongs to the select group of symmetrical sense impressions.2 It is essentially mathematical, and its perception is among the most refined of intellectual achievements. Discord, in contrast, belongs with the less cerebral (and, in evolutionary terms, much older) sense impressions of colour, taste, or touch. We implicitly acknowledge as much whenever we describe a triad as 'bright', a third or sixth as 'sweet', or a diminished seventh as 'velvety'.
Both discord and dissonance contain an element of the subjective. As has often been pointed out, the chord that is startling in Mozart may be soothing in Wagner.
1 Or, of course, multiples of the octave. In the same way, most generalizations about simple intervals
hold good also for their compound equivalents, e.g. major tenths behave like major thirds.
2 The word 'symmetrical' is here used in the scientific sense explained in Ch. I.