This article was written for a seminar held on the occasion of the Franklin Institute's 2001 Bower Award and Prize for the Achievement in Science to the author "for his seminal invention of packet switching - the foundation of modern communications networks and, in particular the Internet." It describes the author's work 40 years ago focusing on the rationale creating the key concepts of packet switching. The article considers the development of each of a series of about 20 essential concepts. For example, it examines such subjects as the degree of redundancy to achieve any desired level of survivability; the necessity to chop data streams into small blocks; what information had to be appended to these blocks to allow the each block to find its own way through the network; why it was necessary for each network element to operate at an independent data rate; why all signals had to be converted to digital, and so on. It describes the "why" as well as the "how" of packet switching works.