This paper introduces a novel thesis about mental imagery; namely, that it is grease for the mind’s gears (MGT). MGT is not a vague analogy. Rather, it outlines an important and overlooked higher-order function of mental imagery: that it aids various psychological faculties in discharging their functional roles. MGT is motivated by reflection on converging evidence from clinical, experimental and social psychology and solves at least two conceptual puzzles about mental imagery. The first puzzle concerns imagery’s architectural promiscuity; that is, its ability to interact with diverse psychological faculties and perform very different functions when doing so. The second puzzle concerns how to square imagery’s architectural promiscuity with its psychopathological relevance; that is, imagery’s capacity to be a maintaining cause, and possibly even a partial constituent, of several psychological disorders, including post-traumatic stress disorder, bipolar disorder and major depressive disorder. Mental imagery helps and harms the human mind to extreme degrees and this is something that calls for elucidation. MGT says that instead of facing perplexing heterogeneities here, we instead face a significant unity. In addition, MGT represents an alternative to the currently dominant conception of imagery in the philosophical literature; namely, as a perceptual or perception-like state of mind.