In Lucretius's universe, there are atoms and there is void – completely empty space. Nothing else can be said to exist
Lucretius's stated aim in his six-book poem, De Rerum Natura, is to free us from fear by enabling us to understand Epicurean philosophy, so giving us the rational explanation of phenomena that have previously attracted irrational explanation and thus created fear of the gods in men's minds – the soul, for example, or thunderbolts. Before he gets near either souls (which he explains in book three) or thunderbolts (book six), however, he needs to get down to the basics, and so he spends the first two books of his poem explaining the basic principles of Epicurean atomic physics.
Our starting point is a simple idea: nothing can ever miraculously be created out of nothing. Get that in our heads, he argues, and we will be able to fix our focus on the actual source for every thing on earth, and be clear that that isn't the gods. To prove it, he gives us a ridiculous counterfactual situation: if things could spring up from no seed, men would grow out of the sea or fish from the ground, the seasons would be meaningless and ageing spontaneous (from boy to man in an instant); that doesn't happen, so things must have fixed seeds. He proves the principle that he regards as its companion piece, nothing can ever be destroyed totally into nothing, in a similar way: if nature permitted total annihilation, any given thing could be wiped away to nothing instantly, with the merest touch required to destroy it; that not being the case, instead it must be that the stuff of a thing is, on the thing's destruction, channelled into another thing. The example we are given is a beautiful picture of the sky as father raining on mother earth, and that union leading to the world we see around us. That world does things in order: things have their seeds, and nothing is ever destroyed utterly.
Atoms are imperceptible, so Lucretius must also prove the existence of invisible particles. He appeals to the familiarities of the surrounding world: there are plenty of things we can't see but nevertheless know exist, such as wind and smells; even our drying laundry is proof that invisible particles exist – we can't see the particles of moisture being drawn out, but we can clearly see that they are.
Atoms in the Lucretian universe are accompanied by void – completely empty space, without any particles in it – both outside and within objects. Without it, there would be no motion, because there would be no space without particles in it into which particles could move, and a ball of wool would weigh the same as a ball of lead. Plenty of material objects have void in them, and can be destroyed; but the matter that doesn't have void in it can't be. Atoms and void are complementary: atoms are particles without void in them, and void is space without particles in it.
Atoms and void are it: nothing else in the Lucretian universe can be said to exist. Anything else is either a property of the atoms in a thing – something that can't be taken away from it without that thing ceasing to exist in that form, such as liquidity for water – or an accident: something that happens to collections of atoms and void, which does not have its own intrinsic existence, such as poverty or war. Atoms collide, stick together and form compounds, and that's how the world comes about. As and when compounds lose atoms, the compound starts to decay or else the lost atoms are either replaced (by eating food, for example).
And the stock of atoms is infinite, because the universe must be: for something to be finite, it has to have a boundary (one meaning of the Latin word, finis). Challenging us to find that boundary leads to one of Lucretius' most inspired images: he takes the historic Roman declaration of war – the hurling of a spear into enemy territory – and asks us to re-enact it at the alleged edge of the universe. If it hits something, there must be something more there; if it carries on flying, there can't be a boundary.
These, then, are Lucretius' basic principles, and not just of physics, but of argument: he tells us before he begins his arguments that the thing that will shake out the fear and shadows of our minds will be the external appearance and underlying explanation of nature (naturae species ratioque), and that is what he uses: familiar sights from our everyday life, or ridiculous counter-factual situations presented as the logical consequence of failing to accept his principles. Lucretius is seeking to explain the world around us, and uses a vibrant snapshot of that world to prove his point. The species of our world leads us inevitably to its ratio.