Pulling into a parking spot at work, you realize you have no recollection of the drive that got you there. On reaching the bottom of a page in a book, you are frustrated that you have failed to understand what you just read. In conversation, you suddenly become aware that you have no idea what the person speaking to you has said.
These episodes are symptoms of a distracted mind. You were thinking about a vacation while reading a report or reliving a hurtful exchange with a friend instead of paying attention to the road or conversation. Whether the mind journeys to the future or the past, whether the thoughts that whisked you away were useful, pleasant or uncomfortable, the consequences are the same. You missed the present, the experience of the moment, as it was unfolding. Your mind was hijacked into mental time travel.