That Lucretius and many others did more than simply associate themselves with Epicurus — that they celebrated him as godlike in his wisdom and courage — depended not on his social credentials but upon what they took to be the saving power of his vision. The core of his vision may be traced back to a single incandescent idea: that everything that has ever existed and everything that will ever exist is put together out of indestructible building blocks, irreducibly small in size, unimaginably vast in number. The Greeks had a word for these invisible building blocks, things that, as they conceived them, could not be divided any further: atoms.

The notion of atoms, which originated in the fifth century BCE with Leucippus of Abdera his prize student Democritus, was only a dazzling speculation; there was no way to get any empirical proof and wouldn’t be for more than two thousand years. Other philosophers had competing theories: the core matter of the universe, they argued, was fire or water or air or earth, or some combination of these. Others suggested that if you could perceive the smallest particle of a man, you would find an infinitesimally tiny man; and similarly for a horse, a droplet of water, or a blade of grass. Others again proposed that the intricate order in the universe was evidence of an invisible mind or spirit that carefully put the pieces together according to a preconceived plan. Democritus’ conception of an infinite number of atoms that have no qualities except size, figure, and weight — particles then that are not miniature versions of what we see but rather form what we see by combining with each other in an inexhaustible variety of shapes — was a fantastically daring solution to a problem that engaged the great intellects of his world.

It took many generations to think through the implications of this solution. (We have by no means yet thought through them all.) Epicurus began his efforts to do so at the age of twelve, when to his disgust his teachers could not explain to him the meaning of chaos. Democritus’ old idea of atoms seemed to him the most promising clue, and he set to work to follow it wherever it would take him. By the age of thirty-two he was ready to found a school. There, in a garden in Athens, Epicurus constructed a whole account of the universe and a philosophy of human life.

In constant motion, atoms collide with each other, Epicurus reasoned, and in certain circumstances, they form larger and larger bodies. The largest observable bodies — the sun and the moon — are made of atoms, just as are human beings and waterflies and grains of sand. There are no supercategories of matter; no hierarchy of elements. Heavenly bodies are not divine beings who shape our destiny for good or ill, nor do they move through the void under the guidance of gods: they are simply part of the natural order, enormous structures of atoms subject to the same principles of creation and destruction that govern everything that exists. And if the natural order is unimaginably vast and complex, it is nonetheless possible to understand something of its basic constitutive elements and its universal laws. Indeed, such understanding is one of human life’s deepest pleasures.

This pleasure is perhaps the key to comprehending the powerful impact of Epicurus’ philosophy; it was as if he unlocked for his followers an inexhaustible source of gratification hidden within Democritus’ atoms. For us, the impact is rather difficult to grasp. For one thing, the pleasure seems too intellectual to reach more than a tiny number of specialist; for another, we have come to associate atoms far more with fear than with gratification. But though ancient philosophy was hardly a mass movement, Epicurus was offering something more than caviar to a handful of particle physicists. Indeed, eschewing the self-enclosed, specialized language of an inner circle of adepts, he insisted on using ordinary language, on addressing the widest circle of listeners, even on proselytizing. And the enlightenment he offered did not require sustained scientific inquiry. You did not need a detailed grasp of the actual laws of the physical universe; you needed only to comprehend that there is a hidden natural explanation for everything that alarms or eludes you. That explanation will inevitably lead you back to atoms. if you can hold on to and repeat to yourself the simplest fact of existence — atoms and void and nothing else, atoms and void and nothing else, atoms and void and nothing else — your life will change. You will no longer fear Jove’s wrath, whenever you hear a peal of thunder, or suspect that someone has offended Apollo, whenever there is an outbreak of influenza. And you will be freed from a terrible affliction — what Hamlet, many centuries later, described as “the dread of something after death,/The undiscovered country from whose bourn/No traveller returns.”

Against other things it is possible to obtain security, but when it comes to death we human beings all live in an unwalled city.

— Stephen Greenblatt, The Swerve: How the World Become Modern