Definitions

propter hoc

Post hoc ergo propter hoc

Post hoc ergo propter hoc, Latin for "after this, therefore because (on account) of this", is a logical fallacy (of the questionable cause variety) which states, "Since that event followed this one, that event must have been caused by this one." It is often shortened to simply post hoc and is also sometimes referred to as false cause, coincidental correlation or correlation not causation. It is subtly different from the fallacy cum hoc ergo propter hoc, in which the chronological ordering of a correlation is insignificant.

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. Most familiarly, many superstitious beliefs and magical thinking arise from this fallacy.

Pattern

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 in reverse: Avoiding A will prevent B.

Examples

From Attacking Faulty Reasoning by T. Edward Damer, Third Edition p. 131:

  • 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, has assumed that the tenant's presence has some causal relationship to the furnace's becoming faulty.

From With Good Reason by S. Morris Engel, Fifth Edition p. 165:

  • More and more young people are attending high schools and colleges today than ever before. Yet there is more juvenile delinquency and more alienation among the young. This makes it clear that these young people are being corrupted by their education.

See also

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