Ever wonder how we know things? Lucretius certainly did, and he also recognized that, without a naturalistic account of knowledge, his proposed Cosmos consisting of nothing other than matter and void would be a non-starter. He argues, necessarily, that all knowledge comes through the senses, and accordingly proposes an empirical epistemology that foreshadows the modern scientific method. He addresses the means why which our senses often seem to deceive us, and argues that, even with its uncertainties, the provisional knowledge offered by empiricism is always better then the illusory certainties offered by religion. One of the principle positions that Lucretius takes on in this book is the so-called teleological argument: the notion that some intent preceded our being generally, and our sense organs particularly. He argues instead, correctly, that function emerges from form rather than predating it. In other words, we don't have eyes so that we can see, but rather, we see because we have eyes. The teleological argument, in other words, indicates an inversion of cause and effect. To partially illustrate the point, I offer a bit of an experiment that you can perform on yourselves. This part of the argument anticipates his discussion of evolution, which follows in Book 5. Also notable in this book, on the topic of senses and knowledge, is Lucretius's understanding of sex and love, his non-binary understanding of gender, and his notion, which seems to anticipate Freud, that much of our cultural activity consists of a redirection of erotic impulses.

Oh, and he also has advice about sex positions.