Why did Lucretius choose to write in poetry? The answer lies in his evangelism for both Epicureanism and his own legacy
Epicurus didn't like poetry. He thought it was unclear in comparison to prose, and in his own works used prose, often of a sparse and crabby variety. A wise man will be able to talk about poetry, Epicurus says – indeed, he will be the only one able to do so correctly – but he himself will not write it.
Lucretius, of course, liked Epicurus a very great deal. His master's attitude to verse has therefore often caused headaches for people reading his poem: why, runs the standard question (a favourite in university courses on the De Rerum Natura), write in verse about a philosophical system whose founder was anti-poetry? In a passage that occurs twice in the poem, towards the end of the first book and at the start of the fourth, Lucretius gives us an answer. Children hate the taste of medicine, even though it is, of course, good for them; so doctors often put it in a cup the rim of which is covered in honey. The child tastes the honey in the first instance, and by the time he is glugging down the medicine it's too late. In the same way, we may find Epicureanism difficult to swallow at first, so Lucretius lures us in with some honeyed poetry, and before we know it, we've taken the philosophical medicine as well. It's a picture that isn't particularly flattering either to Epicureanism or to his readers, but it's the rationale Lucretius states for his choice of verse.
But that isn't the only answer. Lucretius may well have been luring us in for our own good, but he was also doing so for his own good. Prior to Lucretius, there has been one attempt (by someone called Amafinius) to explain Epicureanism in Latin, and that was in orthodox (but perhaps unmemorable – it hasn't survived) prose. As Lucretius proudly announces just before he explains about the honeyed cup, no one has done what he is doing before: he is a pioneer, treading a path untrodden, drinking from springs untouched till now, wearing a garland of flowers picked for the purpose for the very first time (imagery that suggests an affinity with the values of Hellenistic poet Callimachus, who inspired another poet of the late Roman republic, Catullus, and many other writers in a range of poetic genres). Poetry is what, in part, makes Lucretius's work special.
A poetic pioneer, treading his own path in the style of one of the greatest literary influences on his age, Lucretius is also, in metre and genre, an epic poet. Before poetry has been revealed to us as the honey on the medicine cup, there is a telling little vignette that might reveal something of Lucretius's attitude to epic.
On the surface, it is designed to combat the idea of reincarnation, via a snapshot of Roman poet Ennius – who was the first Roman to write in epic hexameters. That work, the Annales, is now in fragments, but we know he wrote about the history of Rome, from the fall of Troy to the 2nd century BC, and considered himself to be the reincarnation of Homer.
It is in that connection that Lucretius mentions him: great though Ennius's poetry is, he was mistaken, because he thought there was an underworld, from which he claimed that Homer rose and talked to him. The subject of that conversation, Lucretius says, was the nature of the universe – in Latin, the rerum naturam. When the first Latin epic poet is visited in the night by a vision of the greatest ever epic poet, they try to talk about the very subject of Lucretius's poem – the rerum natura.
Unfortunately, both chose the wrong subject: as Lucretius will go on to prove, the only things we can say actually exist are atoms and void, so we oughtn't to claim that, for example, historical events have an actual existence – they're simply accidental properties of the atoms around when they happened. The example he chooses to illustrate that idea is Homer's subject in the Iliad, and Ennius's starting point for the Annales: the Trojan war. Epic is about the rerum natura, and, as the first to write epic about Epicureanism, Lucretius is the first one to write it properly. The doctor, it seems, gets as much out of applying the honey as the patient does from drinking through it; atomic physics doesn't just bring us happiness, it brings poetic glory. And, by writing about Epicureanism in epic, with Epicurus himself as a sort of epic hero, Lucretius may not have been a wise man, but he gives, in return, a coating of poetic immortality to his chosen way of explaining the world.