E.R. Dodds, Pagan and Christian in an Age of Anxiety: Some Aspects of Religious Experience from Marcus Aurelius to Constantine
(1965; rpt. Cambridge: Cambridge University Press, 1990), p. 3:
The practice of chopping history into convenient lengths and calling them 'periods' or 'ages' has of course its drawbacks. Strictly speaking, there are no periods in history, only in historians; actual history is a smoothly flowing continuum, a day following a day. And even when hindsight enables us to cut it through at a critical point, there is always a time-lag and an overlap.