An idea can attach itself to your mind, becoming so integrated that the roots of it are long lost and all that's left is the actions that are precipitated by it.
You are walking down the street on a bright summer day. Glancing down, you see the intricate layerings of leafy shadows. The shadows are not all the same value. The leafs that are further away cast a fainter shadow. In some places, the brightest shots of light are surrounded with soft shade. Where the branches are thin, the shadows seem to smear away from the direction of the light.
Shadow as a metaphor for experience. Not just any shadow, but the shadows cast by living trees in all their seasons. Events that happened long ago cast the faintest shadow. But in some places the shadows are so thick that the original pattern of leaf and branch is no longer discernible. Experiences that are numerous, related, blend to such a degree that you can no longer see the original incident as separate but only as reinforcing the subsequent, similar experiences.