“The essence of wisdom is to know when to be doing, and when it's useless even to try”
Mary Stewart book The Last Enchantment
Source: The Last Enchantment

The Last Enchantment is a 1979 fantasy novel by Mary Stewart. It is the third in a quintet of novels covering the Arthurian legend, preceded by The Hollow Hills and succeeded by The Wicked Day.
“The essence of wisdom is to know when to be doing, and when it's useless even to try”
Mary Stewart book The Last Enchantment
Source: The Last Enchantment