Sceptics raise doubts about our ability to have knowledge generally, and naturalists use scientific discoveries to question common-sense thinking about the world, language, and the mind. This book replies to these contentions, using a transcendental argument to show that everyday thought constitutes an interlocking system of concepts presupposed by all types of reasoning, including empirical science. Thus sceptics cannot question ordinary belief, or science challenge everyday thinking, without undermining their own legitimacy. In addition to replying to arguments by scientific naturalists in a number of areas, the book presents common-sense thought in detail about reality and the mind. It also considers the circumstances under which religious belief is justified. The result is a contemporary defense of our over-all conceptual scheme giving everyday thought a central place but also accommodating scientific and other forms of thinking.