Mind wandering is a pervasive feature of subjective experience. But why does the mind tend to wriggle about rather than always staying focused? To answer this question, this paper defends the claim that mind wandering consists in task-unrelated thought. Despite being the standard view of mind wandering in cognitive psychology, there has been no systematic elaboration or defense of the task-unrelated thought view of mind wandering. Here, I argue for the task-unrelated thought view by showing how mind wandering reflects a (...) distinctive form of non-vigilant thinking. When the relationship between mind wandering and vigilance is clarified, several objections to the task-unrelated thought view can be defused. Not only does this provide a novel defense of the task-unrelated thought view of mind wandering, but it also presents an account of mind wandering that is compatible with a naturalistic account of subjectivity. (shrink)