Being lost in thoughts of any kind, pleasant or unpleasant, is analogous to being asleep and dreaming. It’s a mode of not knowing what is actually happening in the present moment.
Thoughts themselves are not a problem, but being identified with thought is. Taking oneself to be the thinker of one’s thoughts—that is, not recognizing the present thought to be a transitory appearance in consciousness—is a delusion that produces nearly every species of human conflict and unhappiness.
It doesn’t matter if your mind is wandering over current problems in set theory or cancer research; if you are thinking without knowing you are thinking, you are confused about who and what you are.
- Sam Harris, "Waking Up: A Guide to Spirituality Without Religion"