No one knows the exact order Plato's dialogues were written in, nor the extent to which some might have been later revised and rewritten. The works are usually grouped into "Early" (sometimes by some into "Transitional"), "Middle", and "Late" period. This choice to group chronologically is thought worthy of criticism by some (Cooper "et al"), given that it is recognized that there is no absolute agreement as to the true chronology, since the facts of the temporal order of writing are not confidently ascertained. Chronology was not a consideration in ancient times, in that groupings of this nature are "virtually absent" (Tarrant) in the extant writings of ancient Platonists.