 Unlike other Hellenistic schools, such as those of Aristotle and the Stoics, the Epicureans were not greatly interested in formal logic, but they certainly needed a theory of the formation of beliefs. As far as the ideational content of thinking — that is, the thought of something — is concerned, Epicurus proposed a radically reductive hypothesis: just as sensations occur as a result of thin films emitted by objects that enter the appropriate sense organ, so too some of these simulacra are fine enough to penetrate directly to the mind (located in the chest), and that is how we imagine such objects (e.g., gods). This process is invoked to explain not only dream images, but many kinds of mental impression, including impressions constituting voluntary thought: the latter occurs when we attend to one or another of the exiguous physical films that are continuously floating through the air. (How we manage to attend voluntarily to whichever of these films we choose is not explained in the surviving sources.) Imagining a thing is thus nothing more than picking out the simulacra that have been emitted by it, and which may endure beyond the life of the thing itself (hence we can imagine the dead). These mental images have no privileged status, such as Plato gave to his noetic Ideas or Forms; they are always true, but in this do not differ from the information provided by the senses. Mistakes occur here too when the wrong beliefs are associated with such impressions, for example, that because we have a mental image of a dead person it follows that he or she still exists in a ghostly form. Epicurean physics proves that this is impossible.