I have a rather basic but vague question. Categories, functors, and natural transformations seem to form the first three elements of a series which could be continued indefinitely. Why is it that these three suffice, and that further members of the sequence are almost never required? Similarly, there are (1-)categories and 2-categories, and further members of this sequence can be defined, yet they are rarely needed. I have once seen 4-categories referred to, but only once. Is there some intuitive explanation for this? -- ____ Richard Kennaway __\_ / School of Information Systems Internet: jrk@sys.uea.ac.uk \ X/ University of East Anglia uucp: ...mcsun!ukc!uea-sys!jrk \/ Norwich NR4 7TJ, U.K. ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++