Post hoc ergo propter hoc (Latin: “after this, therefore because of this”) is a logical fallacy that states “Since event Y followed event X, event Y must have been caused by event X.” It is often shortened simply to post hoc fallacy.
A logical fallacy of the questionable cause variety, it is subtly different from the fallacy cum hoc ergo propter hoc (“with this, therefore because of this”), in which two events occur simultaneously or the chronological ordering is insignificant or unknown.
Post hoc is a particularly tempting error because correlation appears to suggest causality. The fallacy lies in a conclusion based solely on the order of events, rather than taking into account other factors potentially responsible for the result that might rule out the connection.
A simple example is “the rooster crows immediately before sunrise; therefore the rooster causes the sun to rise.”
The form of the post hoc fallacy is expressed as follows: A occurred, then B occurred. Therefore, A caused B.
When B is undesirable, this pattern is often combined with the formal fallacy of denying the antecedent, assuming the logical inverse holds: Avoiding A will prevent B.
Hereinafter, a few illustrations: A tenant moves into an apartment and the building’s furnace goes faulty. The manager blames the tenant’s arrival for the malfunction. One event merely followed the other, in the absence of causality. The Brazilian footballer Pelé is said to have blamed a dip in his playing performance on having given a fan a specific playing shirt; after getting the shirt back his performance recovered. The loss of the shirt was given as the reason for his dip, and its return the cause of his recovery. However, it was later discovered the shirt returned was not the original shirt.
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- Damer, T Edward (1995). Attacking Faulty Reasoning: A Practical Guide to Fallacy-Free Arguments (3rd ed.). Belmont, CA: Wadsworth Publishing. p. 131. ISBN 978-0-534-21750-1. OCLC 30319422.
- Top 10: Football superstitions to rival Arsenal’s Kolo Toure by Sandy Macaskill, The Daily Telegraph 25 February 2009