If you gaze at a single leaf on a single tree, you do not see the other leaves. If you face the tree with no intention and do not fix your eyes on a single leaf, then you will see all the many leaves. If your mind is preoccupied with one leaf, you do not see the others, if you do not set your attention on one; you will see hundreds and thousands of leaves.

A Hereditary Book on the Art of War (1632)