Saint-Exupery takes your breath away with the originality of his metaphors.  That peace is greater than the sum of its parts is one thing, but to liken that to minerals assembling in the ground to form a tree shows the complexity of that sum of parts. 

Peace is present when things form part of a whole greater than their sum, as the diverse minerals in the ground collect to become the tree. 

Enjoy another superb definition of peace by the same writer. 

Source: Antoine de Saint-Exupéry, Flight to Arras, trans. by Lewis Galantière (Harmondsworth: Penguin, 1961), p. 75

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