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Post hoc ergo propter hoc (Latin: 'after this, therefore because of this') is an informal fallacy that states "Since event Y followed event X, event Y must have been caused by event X." It is a fallacy in which an event is presumed to have been caused by a closely preceding event merely on the grounds of temporal succession. This type of reasoning is fallacious because mere temporal succession does not establish a causal connection. 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 logical fallacy in which one event seems to be the cause of a later event because it occurred earlier.

Post hoc is a particularly tempting error because correlation sometimes 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."

Pattern

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: believing that avoiding A will prevent B.

Examples

• A tenant moves into an apartment and the building's furnace develops a fault. The manager blames the tenant's arrival for the malfunction. One event merely followed the other, in the absence of causality.
• Brazilian footballer Pelé blamed a dip in his playing performance on having given a fan a specific playing shirt. His play recovered after receiving from a friend what he was told was the shirt in question, despite it actually being the same shirt he'd worn during his poor performance.
• Reporting of coincidental vaccine adverse events, where people have a health complaint after being vaccinated and assume it was caused by the vaccination.

See also

• Apophenia – Tendency to perceive connections between unrelated things
• Affirming the consequent – Type of fallacious argument (logical fallacy)
• Association fallacy – Informal inductive fallacy
• Cargo cult – New religious movement
• Causal inference – Branch of statistics concerned with inferring causal relationships between variables
• Coincidence – Concurrence of events with no connection
• Confirmation bias – Bias confirming existing attitudes
• Correlation does not imply causation – Refutation of a logical fallacy
• Jumping to conclusions – Psychological term
• Magical thinking – Belief in the connection of unrelated events
• Superstition – Belief or behavior that is considered irrational or supernatural
• Survivorship bias – Logical error, form of selection bias
• Surrogate endpoint – Biomarker proxy outcome measure
• Texas sharpshooter fallacy

Bibliography

1. Woods, J. H., Walton, D. N. (1977). Post Hoc, Ergo Propter Hoc.
2. Mommsen, J. K. F. (2013). Wider Das Post Hoc Ergo Propter Hoc - Primary Source Edition. United States: BiblioLife.
3. Woods, J., Walton, D. (2019). Fallacies: Selected Papers 1972–1982. Germany: De Gruyter.

References

1. Grouse, Lawrence (2016). "Post hoc ergo propter hoc". Journal of Thoracic Disease. 8 (7): E511–E512. doi:10.21037/jtd.2016.04.49. ISSN 2072-1439. PMC 4958779. PMID 27499984.
2. "post hoc". LII / Legal Information Institute. Retrieved 2021-08-28.
3. "Correlation vs Causation". KnowledgeSpace. 2015-10-09. Retrieved 2021-08-28.
4. Summers, Jesse S. (24 March 2017). "Post hoc ergo propter hoc : some benefits of rationalization". Philosophical Explorations. 20 (sup1): 21–36. doi:10.1080/13869795.2017.1287292. S2CID 151401300.
5. 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.
6. Macaskill, Sandy (2009-02-25). "Top 10: Football superstitions to rival Arsenal's Kolo Toure". The Telegraph. Archived from the original on 2010-08-26.
7. Manktelow, K. I. (2012). Thinking and Reasoning: An Introduction to the Psychology of Reason, Judgment and Decision Making. Psychology Press. p. 119. ISBN 9781841697413.

• Causal fallacies
• Latin logical phrases