 Historically, mental imagery is thought to be an essential component of imaginings. Aristotle’s phantasia, which is sometimes translated as imagination, is a faculty that produces images (De Anima; see entry on  Aristotle’s conception of imagination;  but see Caston 1996). René Descartes (Meditations on First Philosophy) and David Hume (Treatise of Human Nature) both thought that to imagine just is to hold a mental image, or an impression of perception, in one’s mind. However, George Berkeley’s puzzle of visualizing the unseen (Three Dialogues between Hylas and Philonous) arguably suggests the existence of a non-imagistic hypothetical attitude.