The only reason large, complex artifacts such as automobiles, aircraft, houses, and ships can be constructed by people who individually can only do so much, is that there is a meaningful way to split large tasks into small ones. The large, extremely complex task that is often even impossible for a single person to comprehend fully, can be divided into small parts that can be completed by a single person or by a small group of people, and there is a well-defined procedure by which all those small parts can then be put together to produce the final, large artifact.

Although the notion of splitting a large task into smaller parts is certainly as old as recorded history--if not more so--the scientific analysis of the notion and its application to computer science are recent. The name given to the concept is "modularity," and any design of a large...