Note 1:

The name of the fallacy was coined by philosopher Henri Bergson to emphasise his view that planning is impossible. That is a philosophical view that could only be formed by someone who has not had to survive in the cut-and-thrust of the real world. Indeed, it would require disregard for the continuum of planning, large and small, that Bergson must have engaged in, including eating breakfast, writing his books, and getting to the lecture hall where crowds of people came to hear him speak. That is, such a philosophy is a performative contradiction.

Note 2:

Despite what Bergson was trying to accomplish, retrospective determinism is a fallacy. It’s beyond the scope of this discussion to go into why, but we encourage you to learn about The Butterfly Effect – the sensitive dependence of a chaotic, turbulent process such as the weather (and by extension, human history) upon initial conditions – and to notice that that sensitivity appears to have no lower bound. That is, the purturbation need not be as big as a butterfly and the image of the butterfly was chosen only because it is more vivid than an air molecule or a sub-atomic particle.

Example 1 (of the fallacy):

Tom says: When he declared himself dictator of the Roman Republic, Julius Caesar was bound to be assassinated.

If Tom’s claim has no supporting argument that Caesar’s assassination was inevitable or even likely then it is a fallacy of retrospective determinism. The counter-argument, that assassination was not inevitable, is supported by the many cases of dictators dying in other ways, such as “old age.” Even if Tom gives logical reasons why Caesar’s assassination was inevitable, it is sufficient to find one other dictator to whom the same logical reasoning applies, but who was not assassinated, to disprove Tom’s conclusion and reveal it as a fallacy of retrospective determinism.

Example 2 :

Dick says: The moment he decided to invade Russia, Hitler was doomed. That’s because Russia was far too big for the German army to occupy and because of the terrible Russian winter. That should have already been obvious from Napoleon’s failed invasion of Russia.

Dick’s argument is an example of retrospective determinism because one can find historical counter-examples that refute both of the supporting reasons. The conquest by Alexander (of Persia), Cortez (of Mexico) and Pizarro (of the Incas) all involved vastly outnumbered forces permanently capturing a huge area. In all three cases, the conquerer or his compatriots continued to rule for generations after the conquest. As for the severe Russian winter, it did not stop the successful Mongol invasion by a comparatively small force; in fact, the Mongol cavalry used the frozen rivers as highways.

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