We need to know the world is made of atoms, Lucretius tells us, to stop us being afraid: before Epicurus, man was crushed by his fear of the gods. Man also frets away his life in another needless fear: the fear of death. Lucretius devotes a book to the pointlessness of fearing death and the afterlife, and the main balm for our hurt minds is the explanation of the soul as an atomic and mortal compound.

Our spirit and our mind (anima and animus in Latin) are made of very, very fine atoms (the spirit permeates every part of our body, whereas the mind stays in one place) enabling us to breathe, perceive and move (animating us, in other words). They are composed of breath, heat, air and a fourth, nameless substance, of incredibly smooth, small atoms, that starts the motions of sensation in our bodies. The balance between these different elements of the soul dictates the temperament: lions, who are naturally aggressive, have more heat in their souls than deer, who are naturally timid and so have more air (its chill induces the trembling that we associate with them) and cows, placidly zen, are clearly possessed of souls with air to the fore, temperamentally between lions and deer.

Compounds of different types of atoms, encased in the body and giving the body its sensation, mind and spirit must be mortal. Some of the examples Lucretius gives us for the way the spirit pervades the body and dissipates with it are a bit grim: there is a disturbing focus on severed eyes, and a passage about limbs being severed in war unbeknownst to the now fewer-limbed warrior verges on Monty Python's Black Knight. But the point is made: fine-spun compound that it is, the soul can hardly survive outside the body that it infused. Water cannot survive if the pot it is in is shattered, and mist dissipates in the breeze; so too when the body dies, the soul disperses. That our minds can be affected by illness is another proof: if it suffers similarly to the body in illness, it must do the same in death.

Our mind and spirit, then, are mortal. As for the punishments that myth claims await us in the underworld, these, Lucretius argues, are rooted in everyday human frailty: Sisyphus pushing his rock up a hill only for it to roll back down again, for example, is the perfect image of someone keen for power, who pushes and pushes to reach the top, only to roll straight back down (compare the greasy pole). In fear of earthly punishments, we imagine unearthly torments: fools make a hell here on Earth.

Besides, a life too long lived, Lucretius argues, is a misery in itself: clinging to life is desperate, foolish and greedy. He has a personified Nature rebuke those afraid of death for, among other things, wanting exemption from something that has befallen people much greater (including Epicurus) and selfishly trying to keep hold of the part of the stock of atoms that makes up their bodies and souls, when those atoms are needed elsewhere – the ultimate guilt-trip about recycling. Lucretius compares the time before we were born to that after we die – no one laments that they felt nothing before they arrived on the Earth, so why should they worry about a similar situation when they depart it?

It is not just us frail humans who are mortal: the world around us, too, is going to wither away at some stage. Lucretius's second book deals mostly with atomic motion and compounds, with a large digression inspired by the diversity of life the Earth supports, and so seems to be a book about life. But at its end he gives us a portrait of an Earth already worn and fading, which yields smaller crops than in the past and produces animals a fraction of the size it used to. That book ends with farmers bemoaning the hard life that the waning of the Earth forces upon them as they try to scratch out a living from it. Lucretius returns to the theme in book five: however great and massive, the Earth will, on a single day, collapse and die.

How much reassurance this portrait of universal destruction gives is debatable. How effective Lucretius's arguments for calm in the face of death are is similarly open to challenge. (Thomas Nagel pointed out, for example, that although we don't miss the time we were unalive before birth, when we die, we lose something good, namely life, and we are entitled to miss that.) Reading them, one thing is sure: Lucretius himself is utterly convinced not just of their truth but of their ability to give us the calm we need when reflecting on our mortality.