This book is concerned with the philosophical foundations of a realist explanation of the progress of science. I shall maintain that central to this explanation is the claim that there are many cases where competing or successive scientific theories are about the same things. I believe such a claim to be intuitively plausible. My principal aim, however, is not so much to champion this belief as to set down in a methodical way what the realist's explanation entails. As the reader will presently see, it raises some of the main problems in contemporary philosophy of language. In particular, the realist has to reply to several powerful a priori arguments directed against his position. Whether he can do so, and whether, therefore, he can fully substantiate his explanation of how science progresses, is, I believe, one of the most interesting questions in modern philosophy.

My approach in writing this book has been to try to give some background to the arguments I discuss rather than to assume the reader is fully conversant with them. One reason for this is that any one person's understanding of a complex argument tends to be different from any other person's. The result is that what appears to some to be germane criticism is regarded by others as beside the point. Another reason harks back to my own early years of studying philosophy. I recall finding it difficult sometimes to relate the abstruse argument of the moment to more mundane problems.