Book excerpt: Yes, But How Do You Know? is an invitation to think philosophically through the use of sceptical ideas. Hetherington challenges our complacency and asks us to reconsider what we think we know. How much can we discover about our surroundings? What sort of beings are we? Can we trust our own reasoning? Is science all it is cracked up to be? Can we acquire knowledge of God? Are even the contents of our own minds transparent? In inviting, lucid prose, Hetherington addresses these questions and more, using scepticism to illuminate many perennial philosophical puzzles.