Why ever CSP?

The original theoretical model of Communicating Sequential Processes owed its inspiration to the achievements of Milner, Scott and Dijkstra. It was developed at around the time of the publication of Milner's Calculus of Communicating Systems. Why ever did CSP diverge from CCS?The ESPRIT basic research action 'CONCUR' brought together the proponents of three of the original calculi of concurrency: ACP, CCS and CSP. It was hoped that we would develop a unified calculus, and concur upon its adoption. Why ever did we fail?I would like to share with you the way in which I thought about these questions twenty five years ago. Since then, many excellent comparative studies have been conducted. As a result, we can now see how such theories can best be unified, without assimilation of their distinctive features or compromise of their distinctive merits.