The paradigm shift is a good phrase used in both science and history. It generally means something or event that changes the way the world is perceived. It is common in history books to read about this or that’s rise and then fall from the Roman Empire to Soviet Communism, but is that really a good description? I don’t think it is. Any serious student of history can demonstrate that nothing really comes to a crashing halt in history and almost every change is either evolutionary or that the signs are clear long before an event happens. There are things that have a revolutionary impact, but even then the impact is almost never instantaneous, it takes time for events, and discoveries to have a huge impact on peoples lives and conditions.
For example, the Roman Empire did not die in 476 on the day when Romulus Augustulus was deposed from the throne and killed by the Goths. In fact, in many ways, the Empire is till with us in cultural influence at least. The same can be said for any number of things and events that supposedly fell. The influence of the British Empire is still felt throughout the world today in the number of countries throughout the world that speak English, have inhabitants descended from colonists, or have legal systems based on English Common Law. Ditto the influence of Soviet Communism, or does anyone care to argue that there is not lingering nostalgia for the USSR in Russia or that it’s influence is not still felt from the sheer number of Soviet produced weapons still used in conflicts throughout the world? The Crusades echo down to the present, at least according to Islamist terrorists who equate their supposed struggle with the contemporary West to that of the Medieval Arabs who fought Richard the Lionhearted, St. Louis, and King Baldwin of Jerusalem in Outremer.
The idea of the paradigm shift is convenient for use by historians because it presents a bright shining line that lets them say here something ended and something new began. The uncomfortable truth is that it is almost never that clear cut. This is not to say that dividing line are not useful, at a minimum they provide starting points for study. If for that reason alone it is useful to divide history into segments. The totality of history is impossible for one person to get a grasp of. Personally, I generally confine myself to military history although even military history cannot be studied in isolation, as periods of peace impact military history just as much as events on the battlefield. They are useful but all students of history and history buffs should keep in the back of their mind that most historical dates that define eras are arbitrary and do not necessarily represent a true division except that of convenience.