In his De Rerum Natura, Lucretius strives to scientifically explain several aspects of the natural world. At times, however, his explanations suggest that his philosophical principles precede scientific evidence. This paper examines the relationship between Lucretius’s science and philosophy in general, and his treatment of the human soul more specifically. Based on the Epicurean principle that the fear of death is irrational, Lucretius attempts to prove that the soul is entirely physical, and will therefore cease to exist after death, while accounting for its sentience. He must describe an atomic soul, no matter how complicating this becomes, in order to satisfy the principle that nothing comes after death. This entails describing the soul in the same manner as perceptible phenomena, and for this reason his evidence meets with several obstacles. Lucretius’s scientific explanation for the soul presents compelling evidence that he forms a scientific basis around pre-existing philosophical principles, contradicting the assumption that science predates philosophy.