Greek εἴδωλα (eidōla), singular eidōlon; Latin simulacra (Lucretius)
Thin films of atoms that stream continuously from the surfaces of bodies, keeping their shape. Their arrival at our eyes is what we call seeing; finer images reaching the mind directly account for thought, imagination, and dreams. The theory explains how the senses can report faithfully on objects at a distance.
1. Key Texts
(1) I say then that likenesses of things and thin shapes are emitted from things off their surfaces; these can be called films or even bark, because each image bears an appearance and form resembling the body from whatever object it flows and wanders forth. ... (2) For certainly we perceive even in plain sight that many things throw off bodies, some dispersed loosely like smoke from burning logs and heat from fires, others more closely woven and dense, like the slender tissue which cicadas sometimes doff in summer, and the caul which calves cast off from their surface at birth, and likewise the garment which the slippery snake sheds among thorns - for often we see brambles enriched with these wind-blown spoils of snakes. (3) Since such things occur, thin images too must likewise be emitted from things off their outermost surface. ... (4) These simulacra are borne along with the highest speed and mobility; first because a minute cause deep inside is enough to push and start them, since they are so fine and thin; next, because being of such loose-woven texture they easily pass through anything and as it were filter through the intervening air. (tr. Long & Sedley)