The Basic Framework

Category theory as such plays no real part in this book. However it is both convenient and illuminating to be able to use the language of that theory to some extent. At the very least it saves a certain amount of repetition. In fact all the categories we shall be dealing with are of the type known as concrete, i.e. they consist of sets with additional structure and functions which respect that structure. However there is little to be gained by restricting attention to such categories at the outset.