This chapter deals with the second approach to doing philosophy historically, which I have called the diagnostic approach. This approach is rooted in the fact that philosophical pictures can be deceptive. A picture may be widely accepted: it may serve as the unquestioned starting point for a great deal of our thinking, and we may take for granted that we understand it. But it may have a hidden significance that escapes us. It may have far-reaching effects on our thinking, perhaps negative ones, that we fail to notice. When this happens, we frequently find it necessary to diagnose the picture. We inspect it with a suspicious eye, in the hopes of discovering its true nature and unearthing the ways in which it distorts our thinking. Typically, this involves tracing the picture's origin: examining how it came into existence, how it came to govern our thinking, and what it led us to neglect in the course of doing so. In returning to the picture's origin, we learn how and why it began to deceive us. We may also discover alternatives to it, competing pictures that it supplanted and that have long been overlooked. Diagnosis of this sort often serves as a form of therapy. Pictures deceive us when we fail to understand their true nature or recognize their effects. In other words, pictures deceive us when we fail to reflect on them. Reflecting on how a picture came to deceive us helps to lessen its hold on us.