The early 20th century saw the emergence of a number of theories whose power and utility reside in large part in their generality. Typically, they are marked by an attention to the set or space of all examples of a particular kind. (Functional analysis is such an endeavour.) One of the most energetic of these general theories was that of algebraic topology. In this subject a variety of ways are developed for replacing a space by a group and a map between spaces by a map between groups. It is like using X-rays: information is lost, but the shadowy image ...(100 of 41449 words)