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Post hoc ergo propter hoc (Latin: "after this, therefore because of this") is a logical fallacy (of the questionable cause variety) that states "Since event Y followed event X, event Y must have been caused by event X." It is often shortened to simply post hoc. It is subtly different from the fallacy cum hoc ergo propter hoc (correlation does not imply causation), in which two things or events occur simultaneously or the chronological ordering is insignificant or unknown. Post hoc is a particularly tempting error because temporal sequence appears to be integral to causality. The fallacy lies in coming to a conclusion based solely on the order of events, rather than taking into account other factors that might rule out the connection.
The form of the post hoc fallacy can be expressed as follows:
When B is undesirable, this pattern is often extended in reverse: Avoiding A will prevent B.
"I can't help but think that you are the cause of this problem; we never had any problem with the furnace until you moved into the apartment." The manager of the apartment house, on no stated grounds other than the temporal priority of the new tenant's occupancy, holds that the tenant's presence has some causal relationship to the furnace's becoming faulty.
In the Dinosaur Comics comic titled Post Hoc Ergo Propter Hoc, T-Rex points out this logical fallacy committed by Utahraptor.
Tim Minchin explains this phrasing in his 2010 comedy special "Ready For This."
The thirteenth episode from the sixth and final season of "Crossing Jordan" uses "Post Hoc, Ergo Propter Hoc" as the title of the episode.