Stoic ontology 101: the difference between reality and existence

[This article goes well with my Stoic Epistemology 101 and Stoic Psychology 101. For this entry, I am indebted to John Sellars, and specifically chapter 4 of his excellent book, Stoicism.]

Is there a difference between what is real and what exists? I bet most people would be somewhat puzzled by that question. Surely what exists is real, and vice versa! Welcome to philosophy, folks, where things aren’t as obvious as it may at first appear, often in unsuspected and interesting ways.

The ancient Stoics were monists and materialists (or physicalists), meaning that they thought that everything that exists is made of one kind of stuff. They were, of course, incorrect about precisely what this “stuff” is. The Stoics thought that the basic constituent of the universe was something they called pneuma (breath), which comes in four modalities (or tensions): cohesion, which is what gives unity to a physical object; nature, which makes some things alive; soul, which gives perception and movement to animals; and logos, which gives reason to human beings. So, for instance, rocks have cohesion, but none of the other three modes of pneuma. Plants have both cohesion and nature, but not soul or logos. Animals have the first three, but lack logos. We got all four (lucky us!).