This book is an exploration and popularisation of ‘active externalism’ or the ‘extended mind’ hypothesis (Clark, 1997, Clark and Chalmers, 1998, Dennett, 1996, Donald, 1991, Hutchins, 1995). I begin with some background about active externalism and the two principal arguments for holding it: the parity argument and the complementarity argument. This book develops the complementarity argument. I think that both arguments are problematic. But I go on to suggest that this fascinating book provides us with preliminary grounds for endorsing a version of active externalism that is stronger and stranger than the one Clark actually defends. Active externalism is the belief that mind extends into the world, beyond the skin-and-skull boundary. Clark and Chalmers (1998) say that cognitive processes extend into the world when we use pen and paper to work something out, or when we use a computer, or even when we use language, which Clark thinks was the first technology. Also, cognitive states extend into the world when we use physical objects, or data-structures such as chips or CD-ROMs, as external memory stores.