This book is an attempt to understand philosophical inquiry by means of answering five questions: What is the structure of theoretical inquiry? What are the goals of theoretical inquiry? What are philosophical data? What is a sound method for philosophical theorizing? Is there reason to think that philosophical progress can be or has been made? Chapter 1 tackles the first two questions: inquiry is organized around the collection of data and the application to data of a sound method, and is ultimately aimed at achieving understanding. Chapters 2 and 3 address the third question, canvassing various theories of philosophical data before landing on an epistemic theory according to which data are what inquirers, considered collectively, have good epistemic reason to believe. Chapters 4 and 5 tackle the fourth question, identifying three data about sound method, surveying and critically assessing prominent extant methods, and defending a novel philosophical method—the Tri-Level Method—by revealing the various ways in which it can handle the three data and facilitate understanding. Chapter 6 addresses the fifth question, arguing that the Tri-Level Method can explain how to make philosophical progress, that its constituent criteria can be used to assess whether such progress has been made, that there really has been substantial philosophical progress, and that philosophy’s progress is relevantly comparable to that made in other fields.