The
Meditations were originally published in a single volume with six sets of
Objections and Replies.
Descartes arranged to have the manuscript for the Meditations circulated among some of the leading intellectuals of the day, so that he could read and respond to their comments on the work. The objections and Descartes' replies to them are fascinating reading -- if you think you've got an original argument against Descartes, check out the objections, because chances are, one of these guys got to it first. Also, while Descartes' replies are sometimes very insightful, they're also sometimes outrageously cocky and dismissive, so they're instructive from a "get to know your philosopher, warts and all" perspective too.