The unpredicable and miraculous differences, then, in the total responses which slight changes in the arrangement of stimuli produce, can be fully accounted for in terms of the sensitiveness of the nervous system; and the mysteries of ‘forms’ are merely a consequence of our present ignorance of the detail of its action. We have spoken above of the ‘elements’ of a form, but in fact we do not yet know which these are. Any musical sound, for example, is plainly complex, though how complex it is from the point of view of its musical effects is still very uncertain. It has pitch, it has _timbre_, the characters which change as it is played upon one kind of instrument or another, the characters which are sometimes called its colour. Its effects also vary with its loudness and with its volume. It may be far more complex still. Its relations again to other musical sounds may be of at least three kinds: pitch relations, harmonic relations and temporal relations, complicated, all of them, in the utmost degree by Rhythm. Possibly other relations still are involved. There would be no advantage here in entering into the detail of the analysis of these qualities and relations. The one point of importance for our present purpose is the immense scope for the resolution, interinanimation, conflict and equilibrium of impulses opened up by this extraordinary complexity of musical sounds and of their possible arrangement. It is not in the least surprising that so few invariable correspondences between stimuli and total responses have as yet been discovered.