 Why study these ancient thinkers? To those who hold that “no conception can be properly understood except through its history” (Comte 1934: 2), the answer will seem obvious. But even those who prefer to engage in philosophical reflection, rather than studying its history, may find these thinkers illuminating. Empiricists have always had difficulty giving an account of key aspects of both everyday and scientific knowledge. They have found it hard to explain how we could know about logical and mathematical truths, causal relations, and (more generally) the modal structure of the world (how things could be). Insofar as we are still faced with these questions, we may be able to learn from the answers our predecessors gave.