Why did Lucretius argue that the soul dies with the body?

Lucretius argued that the soul dies with the body because he viewed the soul as a material entity composed of exceedingly fine atoms. Following the Epicurean tradition, he maintained that the soul is bipartite, consisting of the anima (which provides sensation throughout the body) and the animus (the seat of consciousness located in the breast).1

Because the soul is a physical system of atoms, it is subject to the same laws of nature as all other material things: it is born and grows alongside the body, and upon the body's death, it is inevitably dissipated like "smoke". Lucretius posited this mortality to liberate humanity from the fear of death and the threat of eternal punishment, which he believed were primary sources of human unhappiness. By demonstrating that the soul is not immortal, he sought to establish that "death is nothing to us," as there is no existence—and therefore no capacity for suffering—once the atomic structure of the individual has dissolved.12