Everything we see is made up of the infinite atoms that swirl about us. These perceptions are the basis of our certain knowledge about the world
How do we know that we're real? How do we know that we know? Individuals with our own sets of sense organs and therefore our own sets of sensory perceptions, we can't verify every day that the wall we think is green actually is green, for example, by looking at it through someone else's eyes. Having some means of certainty about our knowledge – defining a criterion of truth – has seemed necessary (if vexing) to philosophers for millennia. The Epicureans had their own argument for the guaranteed truth of our knowledge of the world around us: all perceptions are true.
Lucretius covers this in book four of the de Rerum Natura (On the Nature of the Universe), which deals with a panoply of things connected to images, from how we see, to dreams, to mirrors, to mythological portents. The need to explain how images are formed is linked, like much else in the de Rerum Natura, to human fear: visions of the dead make us think that there are actual ghosts, and so we terrify ourselves when both awake and asleep. In fact, all images, whether of existing or non-existing things, are (of course) made of the infinite atoms that swirl around and within us. Streams of surface atoms fly off objects and enter our eyes to enable sight. They are fine, travel at very high speeds and come at us from all angles all the time, but only the eyes are capable of receiving them. We perceive these, and those perceptions are the basis of our certain knowledge about the world.
We might spot a problem here: images seem to deceive us. The classic example, often put as a challenge to Epicureans, was an oar that looks bent when in water but which we know to be straight (compare a straw in a drink); another was the fact that from a distance, a tower that is square looks round. Lucretius gives us the counter-argument: the perceptions are true – the atoms in the images strike our eyes in the configurations that suggest bent oars or roundy towers – but the perceptions need an interpreter, namely, the mind. If that goes wrong, we can hardly blame the perception: the atoms that form an image of the distant square tower will, by the time they hit our eyes, have been battered by the air they travel through enough to make the image seem a bit rounded, but that isn't an invitation to assume that the tower must also be. We shouldn't blame the eyes for the mistakes made by our minds.
It isn't just images that are atomic, of course; sounds are – echoes make us think there are mountain nymphs – and smells, too. And thoughts are caused by atoms. The mind's speed of thought is amazing because the images that enter only the mind are made of atoms that are far finer than those that make up visual images. The blizzard of images that fly around us explains our ability to think of things that aren't real: some thought-images are formed from atoms that chance to bump into each other in such a way as to make the shape of a long-dead relative; some combine (being made of the same subtle films of atoms, they can do so easily if temporarily), so that a horse image and a man image might make an image the shape of a centaur. Those random images then enter our brains to make us think of a man-horse or a phantom. But again, wise interpretation will save us from error: the thought of a centaur is no reason to assume the external reality of one (especially once we've read book two of the de Rerum Natura and know that atoms can't combine into an actual, tangible centaur). As for ghosts and dreams about the dead, given the infinity of atoms and the multiplicity of images, it should hardly be a surprise if, on occasion, enough of the right ones collide into the right formation to give us a nanosecond's mental glimpse of an image akin to our long-dead aunt Aida.
Nor is it only the fleeting atomic images that can lead us to deceive ourselves: in a slightly bizarre finish to the book, Lucretius deals with the deceptive nature of love and desire. An explanation of wet dreams leads to a lengthy reflection on sex in general: as a good Epicurean, Lucretius puts forward the idea that no sex is best, but, if you need so much that its absence is making you unhappy, no-strings flings are better than committed relationships, which inevitably lead to emotional distress. Physics-based explanations of things such as orgasm and infertility are side-by-side with a satirical rant about how men deceive themselves about their girlfriends' shortcomings. Epicureanism can save us from bad dates, as well, it seems.