In this lecture, Professor Colin McGinn deciphers what we mean when we say we 'know' something to be true. He traces the history of philosophical skepticism, giving the listener several historical arguments for the theory of knowledge, and also touching on arguments made by the likes of Plato, Descartes, and other important philosophers. This lecture is a foundational exercise for the rest of the program, with McGinn arguing that for someone to begin thinking philosophically, he or she must first understand the methods of thinking that go into the pursuit of true knowledge.