How Can I tell why I do one thing, or why I do not do another?

The Penumbra said to the Umbra. "At one moment you move: at another you are at rest. At one moment you sit down; at another you get up. Why this instability of purpose?" "Perhaps I depend," replied the Umbra, "upon something which causes me to do as I do; and perhaps that something depends in turn upon something else which cases it to do as it does. Or perhaps my dependence is like (the unconscious movements) of a snake's scale or of cicada's wings. How can I tell why I do one thing, or why I do not do another?"