A Voice from the Past - Chesterton

This, therefore, is our first requirement about the ideal towards which progress is directed; it must be fixed. Whistler used to make many rapid studies of a sitter; it did not matter if he tore up twenty portraits. But it would matter if he looked up twenty times, and each time saw a new person sitting placidly for his portrait. So it does not matter (comparatively speaking) how often humanity fails to imitate its ideal; for then all its old failures are fruitful. But it does frightfully matter how often humanity changes its ideal; for then all its old failures are fruitless.

G. K. Chesterton, 1874-1936, Orthodoxy, VII. The Eternal Revolution (Dover Publications, 2004, p.101)

Does this make sense? How do we distinguish the ideal from the incidentals that may need to change?