By all accounts, Gaius Memmius was not a very nice man. After being accused of electoral fraud in Rome, he retired to Athens in about 54 BCE, where he wrote erotic poems. On arrival, he bought a plot of land on which he planned to build a house. According to Cicero, the land contained the ruins of the school founded by the philosopher Epicurus, which Memmius intended to level to provide space for his new villa. At the time, Athens was far from being the philosophical centre it had been in previous centuries. In 88 BCE, the Roman general Sulla had crushed a revolt in Greece and closed the philosophical schools on suspicion of political sedition. Still, within a few decades, the philosophers were beginning to return, and rich Romans such as Cicero would send their sons to Greece to acquire an education.

It’s very possible that the property development by Memmius provided the occasion for the composition of one of the most famous poems of antiquity, On the Nature of Things by Lucretius. The Epicureans, Lucretius among them, were outraged that the site of their founder’s school was going to be turned into a retirement home for an amateur pornographer. Lucretius’s poem, dedicated to Memmius, was an attempt to convince the boorish Roman that he was obliterating a site of importance.

To make sure that Memmius understood him, Lucretius wrote in Latin, rather than Greek. The poem sets out the vision of the Universe according to Epicurus, who thought that everything was made up of an infinite number of tiny particles called atoms. However, expressing the doctrines of a subtle philosophical system in Latin was no easy task. Lucretius found that the words for many concepts simply didn’t exist, and he was not always keen on simply transliterating Greek terms into Latin. In particular, although On the Nature of Things is celebrated for its defence of atomism, the poem never uses the word ‘atom’. In Greek, a-tom means un-cuttable because an atom is defined as something that can’t be split into any smaller body. Spurning this etymology, Lucretius instead used native Latin words that literally mean ‘seeds’, ‘bodies’ and ‘first beginnings’.

In recent years, Lucretius has been the subject of bestselling books such as Stephen Greenblatt’s Pulitzer Prize-winning The Swerve (2011) and Matt Ridley’s The Evolution of Everything (2015). Both authors undoubtedly admire Lucretius. Ridley treats him as a proto-scientist whose rejection of superstition and miracles is an example we would do well to follow. Greenblatt credits the rediscovery of Lucretius’s poem in the 15th century to kickstarting the Renaissance. But their enthusiasm for the poem’s apparent modernity has blinded them to its ancient pedigree. For instance, although both Greenblatt and Ridley have surely given On the Nature of Things a close reading, neither of them notes the most startling thing about the cosmology that the poem espouses: its author thought that the Earth was flat. This is so incongruous with the view of Lucretius as an ancient writer who anticipated some essential elements of modern science that it has been largely forgotten. Indeed, you can read a standard reference book such as The Cambridge Companion to Lucretius (2007) from cover to cover and be none the wiser.

Admittedly, Lucretius never explicitly states that ‘the Earth is flat’. You need to understand the context of his ideas to realise that he believes it. Since the foundation of Epicurean natural philosophy is that everything is made up of tiny particles falling within a limitless void, that means there is no beginning or end to space or time, nor anything beyond the material world. Towards the end of book one of On the Nature of Things, we find Lucretius defending his contention that the Universe is infinite. He presents the splendid argument that, if it had a boundary, then all the falling atoms would collect at the bottom. From this, it is clear that Lucretius respected the intuitive idea that there are absolute directions of up and down. By itself, this makes a globular Earth unfeasible because, unless perched right on the top, we’d slip off the sides.

Lucretius then explains that it’s an error to think that the Earth is at the centre of the Universe because, as the Universe is infinite, it can’t have a centre. He’s arguing against ideas found in Aristotle’s lectures On the Heavens, which contain the earliest detailed arguments for a spherical Earth. Writing in the mid-300s BCE, Aristotle said that the Earth sits at the centre of the Universe, to where all heavy matter naturally travels. So, for Aristotle, falling downwards means moving towards the centre of the Earth, while for Lucretius is means drifting in an arbitrary linear direction. It’s immediately clear why Aristotle thinks that the Earth is round, since heavy objects fall towards it in all directions. In contrast, Lucretius ridicules the idea that anything could be on the other side of the world because it would fall off into the vastness of space. He didn’t just reject a spherical Earth, he thought it was daft.

In a way, he was right about that. Although we condemn flat-Earth thinking as an example of foolish ignorance, a spherical Earth is actually counterintuitive. It’s such a radical idea that it has been ‘discovered’ only once, in Athens after 400 BCE. The concept of the Earth being round didn’t appear in any other civilisation. India and the Islamic world learnt it from the Greeks, while China had to wait until the Jesuits arrived in the 16th century and turned the Chinese view of the Universe upside down. Still, at the time that Lucretius was writing in the 50s BCE, educated Greeks and Romans seem to have been familiar with the true shape of the Earth, so the flat-Earth cosmology of On the Nature of Things was quite eccentric for its time. The reason that Lucretius spurned educated opinion and rejected a spherical Earth becomes clear when we realise that the worldview of his poem is based on source material that was written two and a half centuries earlier by Epicurus himself, the man who founded the school that Memmius was planning to build over.

When Epicurus was writing in about 300 BCE, the shape of the Earth was still up for debate. Since flatness was consistent with the rest of Epicurus’ philosophy, that was what he went with. And this wasn’t his only cosmological infelicity. We know from his surviving letters that he also thought that the Sun and Moon are rather small and nearby. By downgrading the heavenly bodies to mere atmospheric phenomena, he was stripping them of their divine status and showing that it is pointless to worship them. Since Lucretius follows Epicurus slavishly, it is no surprise to find that, in book five of On the Nature of Things, he also tries to convince us that the heavenly bodies are not very big or far away, and that they can even be buffeted about by the winds. He proposes that the world is rather like a snow globe. We are all living on a flat surface covered by a rounded vault, within which the stars and planets move around like the flakes of white when the snow globe is shaken. The whole contraption is falling through the void and will eventually decay into its constituent atoms. It is quite possible, says Lucretius, that the infinite space of the Universe contains many other worlds each with their own heavens and Earth.

The entire Epicurean world-picture buttresses an ethical framework that provides a way of achieving happiness

Why did Lucretius ignore the work of contemporary astronomers such as Hipparchus and Aristarchus, who knew that the Earth is a sphere, the stars are distant and the Sun is much larger than the Earth? It was because Lucretius wasn’t really trying to reveal the truth about the physical world. His purpose, like that of Epicurus, was ethical. Epicurean philosophy teaches that the key to happiness is to avoid pain and experience pleasure. A key source of human angst is fear of death, and what punishment might come after it, as well as the suffering that can be inflicted on us by the gods. By explaining that the Universe is wholly material and that the gods are not bothered about human beings, Epicurus tried to banish fear of the supernatural. If we think we understand how the world really works, we will no longer be afraid. It doesn’t even matter if our beliefs about nature are false. Epicurus is explicit on this point: a completely accurate physical theory is beside the point. His natural philosophy is intended only to justify his ethical belief that the wise man has nothing to fear.

However, Epicureans also had a commitment to free will, which made materialistic atomism problematic. After all, if we are made of tiny particles bumping into each other and occasionally sticking together, it is hard to see where our ability to make choices comes from. Lucretius explains that some atoms occasionally swerve in an unexpected direction, which undermines the determinism implicit in atomism. Even among his admirers, the swerve is seen as an ad hoc explanation. However, it is simply the most obvious of the ways that Epicureans moulded their theory of nature to their ethical beliefs. By focusing on the swerve, we lose sight of the fact that the entire Epicurean world-picture buttresses an ethical framework intended to provide humans with a way of achieving happiness.

It turns out that Ancient Greek science was almost always working in the service of a systematic philosophy. Philosophical schools saw their mission as more ambitious than simply being lovers of wisdom (the literal meaning of philos-sophia). They tried to promote a way of life that could lead their adherents to happiness, or at least contentment. To do this, they developed holistic systems of thought purporting to account for all aspects of life. For example, Aristotle taught that everything in nature has a purpose. The key to happiness is to know your place, whether that is being a slave or a philosopher. The Stoics, another philosophical school, also had a distinctive natural philosophy: they believed that nature was infused with a rational spirit that they called ‘god’. Human beings find happiness by living in accordance with nature, which was how Stoics defined virtue.

While philosophers might disagree about ethics and the best way to live, there were few moral relativists in Ancient Greece. Epicurus, Aristotle and Zeno, the founder of the Stoics, not to mention Plato, all believed that their moral values were objective truths, not simply opinions. This meant that the concepts of right and wrong needed to be grounded in something external to the human mind. For Plato, this was the form of the good, a transcendent exemplar of all that is right. The Stoics believed that virtue was written into the very fabric of reality, waiting to be discovered by the wise sage. It followed that once you discovered how to be good, you had also discovered quite a lot about how the Universe worked. Ethical precepts came first, and natural philosophy had to cohere with these moral commitments. In short, Epicurus did not derive his ethics from his atomism. He believed in atoms because they provided a theory of nature that supported his ethics.

The primacy of ethics over objective knowledge explains some of the puzzling facts about Greek science. For example, Greek natural philosophers rarely bothered with experiments. They already knew their science was true because it was consistent with their wider philosophy. They also made some obvious mistakes, such as Aristotle’s belief that heavy things fall faster than light ones. He thought that this must be true because the heavier an object is, the more it strives to reach its proper place in the middle of the Universe, that is at the centre of the Earth. Aristotle’s error makes perfect sense once we realise that it is not gravity causing stones to fall but their innate propensity to occupy the station ordained for them by the order of nature. Thus, a spherical Earth was evidence that the Universe has a centre, which implies that it is finite, which means that there is room for god beyond its outer edge keeping the whole cosmos in motion. All of this was anathema to Epicurus and Lucretius, who consequently rejected the whole lot. Epicurus went as far as trying to besmirch Aristotle’s reputation by spreading scurrilous rumours that he squandered his inheritance.

All this made progress in science very difficult. If theories about nature depended on theories about morality, and people couldn’t agree about morals, they weren’t able to agree about nature either. The way that Greek science got stuck in a rut is perfectly illustrated by the work of Simplicius, a 6th-century pagan philosopher. Reading his commentaries, we find that the debate about how objects move hadn’t advanced much beyond where Aristotle got to almost a millennium before. Indeed, the first record of someone saying Aristotle was wrong about heavy objects falling faster was John Philoponus, a Christian contemporary of Simplicius. Philoponus thought that right and wrong were decided by a transcendent deity rather than being woven into the fabric of nature, which arguably freed him to pursue a different kind of science. Still, by Simplicius’s time, almost everyone agreed that the Earth is a sphere.

Because the natural philosophies of the Stoics, Epicureans, Academics and the rest functioned as foundations for ethical theories, they had a very different function from today’s science. A modern physicist wants to know how the Universe really works. She is interested in discerning objective truth, not just finding a convenient story that allows her to stop worrying about the gods. Nonetheless, we are closer to the worldview of the ancient philosophers than we might care to admit. We still find it hard to accept that nature is utterly indifferent to what we think. It’s a commonplace that science can’t tell us about right and wrong, but we are unnerved when the physical world does not conform to our instincts about the way things ought to be. Just like the Greeks, we expect our ethical, religious or political commitments to be reflected in the material Universe. Unlike them, we find science’s autonomy means that it doesn’t always reflect our prejudices.

Hume taught that we can’t get ‘ought’ from ‘is’. It’s just as hard to accept that we can’t get ‘is’ from ‘ought’ either

Take creationism, for instance. The story of the six days of creation at the beginning of the book of Genesis has the explicitly theological message that the God of the Hebrews made the world for the benefit of humanity. This story is probably a response to a Babylonian myth, known as Enuma Elish, that implies that the god Marduk created the world for the benefit of the deities of Babylon. The Genesis account underlaid numerous assumptions about the natural world in premodern Europe, such as the expectation that every plant and animal had a medicinal, nutritional or salutary benefit to mankind. Theologians were happy to rise to the challenge of explaining why humans should be grateful for the creation of horse flies and lions. They explained that flies exercised our ingenuity, and lions served as a warning of God’s wrath. Today, science has moved on but creationists continue to see living organisms as products of a divine hand, drawing comfort from this evidence that God is ultimately in control.

Political ideology can also lead people to make demands upon the natural world and challenge scientific results that are morally intolerable. On the Left, gender theorists have refused to countenance evidence of biological differences in the behaviour of men and women. Meanwhile, the way that climate change demands government intervention has made it a hot potato on the Right. Environmentalists are happy to ignore the record on genetically modified organisms because it offends their notions of natural purity. It is easy to generate plenty more examples of groups that are selective about the science they support. We might condemn these ideological responses as evidence of explicit bias, or else claim our political opponents are fools who might as well believe the Earth is flat. That gets humans wrong. Our instinctive ethical beliefs, and the political and religious commitments that develop from them, are quite strong enough to overcome our objectivity. It is perfectly natural to imagine that nature revolves around ourselves and our priorities. The 18th-century philosopher David Hume taught us that we can’t get ‘ought’ from ‘is’. But it is just as hard to accept that we can’t get ‘is’ from ‘ought’ either.

The modernity of Lucretius is not found in his account of a natural world indifferent to humanity. Instead, he expected to find his ethical beliefs reflected back at him when he looked at the workings of nature. Furthermore, he took his conclusions from Epicurus, an authority figure who wrote centuries earlier, and whom Lucretius venerated as the wisest of sages. They both thought that the Earth was flat because this cohered with the totality of their ethical commitments. To admit to a spherical Earth would give rise to a host of questions and throw their most deeply held beliefs into doubt.

Lucretius shows that we have not changed in two millennia. We’re still happy to believe things that conflict with scientific consensus. It just isn’t natural for humans to think objectively. That’s why we find the discoveries of modern science such a challenge and why, even now, we are not willing to accept them without a fight.