Post Hoc Ergo Propter Hoc


The argument offers an explanation, based on a temporal ordering of the events, that confuses co-occurrence with causality: A happened just before B, so A caused B.



The Latin phrase "post hoc ergo propter hoc" means "after this, therefore because of this." The fallacy is generally referred to by the shorter phrase, "post hoc."



"Every time that rooster crows, the sun comes up. That rooster must be very powerful and important!"


"Nearly all heroin addicts used marijuana before they tried heroin. Clearly marijuana use leads to heroin addiction."



Newton thought of cause and effect as sequential: a pool cue must begin moving before it strikes a ball, which must move before it can strike another ball, etc. When objects bump into each other, motion is conveyed from one to another. Since motion takes place in time, cause and effect must be temporally ordered. An effect can happen before the cause only in science fiction stories involving time machines--which is to say, it can't happen in reality (as far as we know).

The fallacy of Post Hoc Ergo Propter Hoc begins with the observation that two events occurred in sequence. As such, it appears to be good Retroductive reasoning, since such a temporal ordering is just the kind of concomitance that might suggest a causal connection. However, it is also possible that the temporal ordering is just a coincidence, or is the result of some further causal factors--indeed, unrelated events occur in temporal sequence all the time. The vast majority of events that happen in one moment are unrelated to the events that happen in the next. ("A bird flew past my window just before my computer crashed. Drat that bird!") Hence, temporal ordering alone is a poor guide to causal relationships. The Post Hoc fallacy mimics good Retroductive reasoning by noting a genuine concomitance, but it errs in focusing on a concomitance that is so commonplace as to be meaningless, unless accompanied by other suggestive details and a common sense understanding of how cause and effect work.


Source: The Port-Royal Logic (Antoine Arnauld and Pierre Nicole, L'Art de Penser, 1662) distinguishes this fallacy from the older Non Causa Pro Causa fallacy.


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