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      The Epicureans, like other ancient philosophical schools, offered a detailed and comprehensive account of physics, including perception.  This branch of philosophy was especially important for Epicureanism due to its crucial role in dispelling fears about the gods, death, and celestial phenomena—fears which Epicureans believed caused mental anxieties and threatened our acquisition of happiness (see e.g. Epicurus, Ep. ad Hdt. 79, Ep. ad Pyth. 85, KD 10-11; Lucretius, 4.33ff, 5.110ff).  Therefore it was necessary for the school to advance a strictly materialist and atomistic explanation of perception and sensation, based, likely to a large extent, on the theories of the Presocratic philosopher Democritus (see Furley 1993).

      Epicurus’ insistence on materialistic explanations and his high standards for empirical verification of his claims yielded an account of perception which is in many ways remarkably close to modern theories.  His theory, which argues that perception is caused by the impact of thin atomic films (called eidola) shed by external objects on our sense organs, and offers criteria for the verification (“witnessing”) of these mental impressions to account for and avoid optical illusions, has been justly praised for its ingenuity and continuing philosophical interest (Long and Sedley 1987: i.78; cf. Everson 1990: 183 and Asmis 2009: 100-104).

      I propose to examine a more surprising and often neglected consequence of the Epicurean theory of perception: its materialistic account of imagination, thought, and dreams.  Epicurus’ Letter to Herodotus and especially Lucretius' account of eidola in De Rerum Natura IV reveal that the school explained these various mental phenomena by analogy with sense perception: our mind is impacted by special, particularly thin and fine eidola, which in turn form the basis of our thoughts and dreams.  I propose to examine the epistemological motivations and coherency of this typically marginalized aspect of their physical system.

      As a point of departure for my analysis I focus on an intriguing critique of this theory made by Cicero in his De Natura Deorum and in a private letter to C. Cassius Longinus written earlier that year (DND 1.107-9; Ad Familiares 15.16).  Cicero’s arguments, which have been alternatively ignored, written off as mere “jokes” (Castner 1988: 30; Lintott 2008: 324; Bailey 1947: iii.1269), or used in the service of Quellenforschung to reconstruct the positions of the Academic Carneades (Kleve 1978: 67, followed by Asmis 1984: 119 n.2), are in fact philosophically sharp and deserve to be considered in more detail.  In these passages, Cicero accepts—for the sake of argument—that Epicurus’ explanation of the five senses is correct and instead focuses his attack on the account of mental perception.  He demands that his Epicurean interlocutors justify the extravagant conclusions of their theory, which would seem to necessitate an infinite availability of eidola of literally everything in every location (e.g. Fam. 15.16: “Is it the case that your [eidolon] is in my power, so that it meets up with me as soon as it pleases me to think of you?  And not only of you, who cling to my very marrow, but if I start to think of the island of Britain, will its εἴδωλον fly into my heart?”). 

      I argue that Cicero is pressing the Epicureans on a very soft spot, and I explore possible Epicurean motivations for this seemingly strange theory.  Drawing upon Lucretius Book IV, I argue that the Epicurean explanation of mental perception connects with two critical assumptions in Epicurean physics and epistemology, both of which Cicero challenges: their claims about the infinity of atoms justify a corresponding infinity of eidola of every object in every location; and their standards of scientific explanation warrant the postulation of these unverifiable and especially fine mental eidola in a way that their more rigorous requirements for explaining sense perception do not.  Cicero’s critiques, then, go much deeper than an attack on a bizarre but minor consequence of Epicurean physics; they intersect with deep epistemological claims about explanation, evidence, and proof.