Thoreau writes in his journal:
Passion and appetite are always an unholy land in which one may wage most holy war. Let him steadfastly follow the banner of his faith till it is planted on the enemy’s citadel. Nor shall he lack fields to display his valor in, nor straits worthy of him. For when he has blown his blast, and smote those within reach, invisible enemies will not cease to torment him, who yet may be starved out in the garrisons where they lie.
