 To have a (merely) mental image is to have a perception-like experience triggered by something other than the appropriate external stimulus; so, for example, one might have “a picture in the mind’s eye or … a tune running through one’s head” (Strawson 1970: 31) in the absence of any corresponding visual or auditory object or event (see the entry on  mental imagery).  While it is propositional imagination that gets compared to belief and desire, it is sensory or imagistic imagination that get compared to perception (Currie &amp; Ravenscroft 2002). Although it is possible to form mental images in any of the sensory modalities, the bulk of discussion in both philosophical and psychological contexts has focused on visual imagery.