 Nevertheless, the notion of a natural property has enjoyed a wide currency in recent philosophy, especially in metaphysics. Lewis develops a rich body of theory in which naturalness plays a central part, and other philosophers have built on and modified Lewis’s ideas in various ways. As we will see in  §3,  naturalness has been invoked in theorising about a wide range of topics, including similarity, modality, laws of nature, mental and linguistic content, ampliative inference, and epistemic value. These roles provide many different ways of getting a grip on what naturalness is supposed to be, and why it might be interesting and theoretically important.