This book discusses a fundamental problem of the natural sciences, namely, the observation of physical and biological systems and the principles by which we gain information about these systems. Is it relevant whether the human observer is himself a part of the system in question? What difference does this make to our representation of the physical world? These difficult questions have occupied scientists and philosophers from Decartes to Bell. In the light of recent advances in quantum mechanics and the theory of dynamical systems, there is good reason to tackle these problems afresh. This book will provide stimulating reading for scientists and philosophers interested in the fundamentals of their trade.