What is it about you in virtue of which you are having the thoughts you are now having? The answer will no doubt make some appeal to the state your brain is now in. Most philosophers, however, claim that this is only part of the answer; many of the facts that determine your thoughts lie outside your skin. This view is called externalism, and in this book Keith L. Butler argues that, contrary to widespread philosophical opinion, externalism is very implausible. Through critical evaluation of a vast amount of philosophical work on the subject, Butler shows that externalism faces problematic epistemological implications (regarding self-knowledge and knowledge of the external world), and problematic metaphysical implications (regarding mental causation). Moreover, externalism derives no support from an appeal to the cognitive sciences. The controversy generated by this book bears on issues in the philosophy of mind, language, science, and epistemology. It is required reading for serious students and professionals concerned with these areas of philosophy.