Book III focuses on the nature of the mind and spirit, establishing the argument that we should not fear death.

Lucretius begins by invoking Epicurus, the philosopher who originated this school of philosophy. Lucretius sets the stage for the following arguments by reminding us that Epicureans believe that fear of death is an evil born of ignorance. Once we understand the mind and spirit, we will no longer fear death.

Lucretius treats the mind and the spirit as similar but distinct parts of the body. The mind is the seat of intelligence, and it resides in the breast. The spirit is under the mind’s control, and it occupies the entire body; it doesn’t reside in any particular part.

Lucretius tells us which types of atom make up the spirit and mind. The spirit has particles of heat, wind, air, and an unnamed fourth. The fourth element, though it seems to defy description, can be seen as “the very soul of the whole soul” (Book III, line 281; page 75). The mind is also made up of heat, wind, and air, and the distribution of these elements in the mind helps to explain different dispositions.