Philosophers have long been fascinated by the stream of consciousnessthoughts, images, and bits of inner speech that dance across the inner stage. Yet for centuries, such 'mind-wandering' was deemed private and thus resistant to empirical investigation. Recent developments in psychology and neuroscience have reinvigorated scientific interest in the stream of thought. Despite this flurry of progress, scientists have stressed that mind-wandering research requires firmer philosophical foundations. The time is therefore ripe for the philosophy of mind-wandering. Our review begins with a foundational question: What is mind-wandering? We then investigate the significance of mind-wandering for general philosophical topics, namely, mental action, introspection, and the norms of thinking and attention.

Although mind-wandering research is rapidly progressing, stark disagreements are emerging about what the term "mind-wandering" means. Four prominent views define mind-wandering as (a) task-unrelated thought, (b) stimulus-independent thought, (c) unintentional thought, or (d) dynamically unguided thought. Although theorists claim to capture the ordinary understanding of mind-wandering, no systematic studies have assessed these claims. Two large factorial studies present participants (N = 545) with vignettes that describe someone's thoughts and ask whether her mind was wandering, while systematically manipulating features relevant to the four major accounts of mind-wandering. Dynamics explains between four and 40 times more variance in participants' mind-wandering judgments than other features. Our third study (N = 153) tests and supports a unique prediction of the dynamic framework-obsessive rumination contrasts with mindwandering. Our final study (N = 277) used vignettes that resemble mind-wandering experiments. Dynamics had significant and large effects, while task-unrelatedness was nonsignificant. These results strongly suggest that the central feature of mind-wandering is its dynamics.

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