Post hoc ergo propter hoc.

 

Post hoc ergo propter hoc (Latin for "after this therefore because of this") is often shortened to just 'the post hoc fallacy'. In its classic form the fallacy was said to occur whenever it was assumed that event B was caused by event A simply because event A was followed by event B. Some people distinguish this fallacy from cum hoc ergo propter hoc (with this therefore because of this) however the issues are mostly the same—the assumption that because two events are correlated one must be the cause of the other..

It is always a fallacy to assume that just because two events are correlated one of the events must be the cause of the other. All causally connected events will be correlated but not all correlated events will be causally connected.

Identifying the post hoc fallacy can be difficult because sometimes it seems reasonable to conclude that A did indeed cause B. If I eat some old leftovers for my supper and then wake in the night violently ill it is not unreasonable to draw a causal connection.

It may be that for good evolutionary reasons we are predisposed to assume that correlation implies causation however we will often pause to consider the plausibility of the causal connection. If I sneeze and the lights go out I may fleetingly entertain the idea that my sneeze somehow caused the lights to go out. If I stop there then I would be committing the fallacy. However, it is more likely that I will laugh at the thought and reject it. I have enough background knowledge of the world to suggest such a connection is unlikely. On the other hand if I turn on the lights and suddenly all the power goes off the casual connection is a lot more plausible. I have enough experience to know that switching on the power can sometimes 'trip' a circuit breaker which may then have a wider effect. Whether the fallacy has been committed seems to depend on how much the reasoning depends solely on the correlation and whether due consideration has been given to the background conditions and other possibilities.

The post hoc fallacy may be understood as a failure to properly consider the plausibility of the causal assumption.