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 fallacy. It is subtly different from the fallacy cum hoc ergo propter hoc (“with this, therefore because of this”), 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.
he following is a simple example:
The rooster crows immediately before sunrise;
therefore the rooster causes the sun to rise.
The form of the post hoc fallacy can be expressed as follows:
- A occurred, then B occurred.
- Therefore, A caused B.
When B is undesirable, this pattern is often extended to the inverse:
Avoiding A will prevent B.
- “I can’t help thinking 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.
- 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. Thus, he said, the loss of the shirt was the reason for his dip, and its return was the cause of his recovery. (It was later discovered that the shirt that was returned to him was not, in fact, the one he had given up.)