This book lays out Epicurean cosmology and the nature of our world. Its arguments can be divided into the birth of the world, astronomy, the birth of life on Earth, and the origins of civilization.

Lucretius introduces his detailed cosmology by announcing that the ‘world,’ by which he means the sky, the sea, and the land, will collapse one day. Though this is inevitable, he does express the hope that “reasoning rather than reality [will] convince you that the whole world may give way and collapse with a horrendous crash” (Book V, lines 108-109; page 139). Lucretius also reminds us that the combination of chance and time created the world: particles traveling at random were bound to create these circumstances eventually. The gods, therefore, did not have a hand in the world’s creation.

The main elements that make up our world are earth, water, air, and fire. Lucretius shows that the world must be mortal by demonstrating how all of its component elements are mutable and weak (“soft,” as established in Book I); evidence that the world will collapse, since something that is made of mortal components cannot itself be immortal. The birth of the world was chaotic, according to Lucretius, with the four elements in imbalance and its component atoms forming a disordered cloud.