The Earth, according to Lucretius, is an amazing thing. It supports a huge variety of life and can provide us with everything we need for happiness, provided we realise that relaxing by a stream in the sunshine trumps inlaid ceilings and fancy banquets. We might, then, be tempted to think ourselves special or blessed in some way, with a world designed to be full of goodness and a body designed to enjoy the fruits of that goodness.

Not so, however; as Lucretius makes clear on more than one occasion, neither our world nor we ourselves were designed. Sense organs existed before the sensory perceptions they are used for; how could seeing exist before there were eyes, after all? We were not purpose-built by the gods: before our creation, without a model to follow, how could they have come up with the idea? Instead, the never-ending random collisions of atoms eventually created complex compounds, such as our world and ourselves; any functions came later.

If the gods had designed our world for our use, they did a pretty poor job, after all: swaths of it are uninhabitable desert and sea, it is full of animals out to eat us and it is going to die one day. Lucretius closes book five of De Rerum Natura with a lengthy description of the history of the world and the evolution of mankind in which he underlines his point, arguing against divine creation of anything and on several occasions emphasising the decay of the Earth from its highly fertile original condition: the popular myth of a golden age, when milk flowed from the Earth and there was abundant food for everyone is used to emphasise the mortality of our world. He has already shown us that, the universe being infinite, our world can't be the only one: the other atoms outside our world must have collided together to form other worlds beyond our own. The gods could hardly be expected to design and govern every single one of those worlds, all at the same time, and remain the blessed beings that we perceive them as and that they must be if they are to be gods.

This principle underlies Lucretius's main argumentative tactic when, in his final two books, he explains the sort of celestial, terrestrial and meteorological phenomena that men in the past have attributed to the gods. Epicureans held that, provided an explanation of a phenomenon is reasonable, it will be true, somewhere in the infinite universe. Lucretius offers us a range of explanations – the stars and planets move in the sky either because the sky moves through air flows, or else the sky is stationary and the stars and planets themselves move, either spontaneously or driven by currents or ether or air. We might not be able to tell which is the case in our world, but provided the explanation is reasonable and we don't forget that the cause is never divine intervention, we don't really need to mind: on the bigger questions, Epicureans operate very much on a need-to-know basis.

The final phenomenon Lucretius explains in his poem is plague and illness. Having given us a few reasons as to why plagues arise and then settle on certain areas, he gives us a detailed account of the one that devastated Athens in the 5th century BC, during the Peloponnesian war (his poetic account draws heavily on Thucydides's prose history of that war). It is a gruesome picture of horrific symptoms, agonising death and social disorder – there are so many corpses to burn that the relatives of the dead fight over funeral pyres – and it is also the end of the poem as we know it. That has caused much scholarly debate: the grimly abrupt ending has led to the argument that Lucretius would have supplied us with a final concluding few lines, post-plague, that he didn't get round to writing for some reason. But another, perhaps more sympathetic, reading of the sudden ending has been proposed, based on Lucretius' mission. We have spent the past 7,500 lines being equipped with all the arguments we need to live a happy life even when looking death and suffering in the face. So Lucretius gives us a challenge: finish his poem with a mind that remains focused on the physics and reality of the world, and you know you've learnt his lesson. At the start of the second book, he tells us that it is sweet to look at someone's ship in trouble on the sea, not because people enjoy watching suffering, but because you are grateful to be safe yourself. Perhaps the poem's ending is a test of that notion; and if you haven't learnt how to do that yet, you should turn back to the beginning and read it all over again.

• This piece was launched a week early in error. There will be no How to Believe next week.