This paper introduces a novel conceptualisation of mental imagery; namely, that is grease for the mind’s gears (MGT). MGT is not just a metaphor. Rather, it describes an important and overlooked higher-order function of mental imagery: that it aids various mental faculties discharge their characteristic functional roles. MGT is motivated by reflection on converging evidence from clinical, experimental and social psychology and solves at least two neglected conceptual puzzles about mental imagery. The first puzzle concerns imagery’s architectural promiscuity; that is, (...) its ability to assist diverse mental faculties and perform many different functions when doing so. The second puzzle concerns how to square imagery’s architectural promiscuity with its psychopathological relevance; that is, its being 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 human psychology 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. On this score, MGT is argued to be superior to the currently dominant conception of imagery in the philosophical literature; namely, as a perception-like state of mind. (shrink)