Is matter “an idea in the mind of God,” as the early modern philosopher and noted immaterialist George Berkeley would have it? On this week's episode of “What Is X?” Justin invites on Sean Carroll, a theoretical physicist at Caltech and host of the podcast “Mindscape” to try to improve on Berkeley’s original definition. But this is not so simple a task. After all, asking “What is matter?,” as Carroll notes, raises a bigger and more salient question—namely, is there matter at all? Over the course of their wide-ranging hourlong conversation, Justin and Sean discuss everything from the fundamental ontology of quantum mechanics and human consciousness to scientific literacy and why crackpots love the theory of relativity.