We have increasingly powerful reasons for acquiring some understanding of the natural sciences. Their influence on the technologies that shape our lives has already been immense, and undoubtedly will continue to grow. In peace and in war, in work and in leisure, in health and in sickness, in each of the different stages of life, we cannot escape that influence. This book is being written with the aid of an electronic computer of a type which, as little as twenty years ago, was unavailable and unimagined by most people. You could well be reading it in circumstances equally unanticipated. On the surface at least, the most prominent differences between our lives and those of earlier generations are differences which have come about as a result of discoveries, investigations, explorations and inventions in the natural sciences. If we compare our modes of transport or communication with those available to previous generations, or compare our education with theirs, we cannot help but be struck with the consequences, for good or for ill, of scientific knowledge. On the credit side, that knowledge, but not that knowledge alone, has resulted in such benefits as the elimination of drudgery and repetitive work for some people, the eradication and control of some life-threatening diseases, and increases in crop productivity. For the sake of these and other benefits we have welcomed science. But we also fear science because, on the debit side, scientific knowledge, though not scientific knowledge alone, is responsible for such harms as the damage suffered by our environment, and has led to questionable experimental practices which need the control of so-called 'ethics committees'. Without the scientist's knowledge of theories, of laws, of techniques and, in general, of what is possible and what is not, the circumstances in which we live our lives would undoubtedly be different.