Page "Correlation does not imply causation" Paragraph 24

Intuitively, causation seems to require not just a correlation, but a counterfactual dependence.

Suppose that a student performed poorly on a test and guesses that the cause was his not studying.

To prove this, one thinks of the counterfactual – the same student writing the same test under the same circumstances but having studied the night before.

If one could rewind history, and change only one small thing ( making the student study for the exam ), then causation could be observed ( by comparing version 1 to version 2 ).

Because one cannot rewind history and replay events after making small controlled changes, causation can only be inferred, never exactly known.

This is referred to as the Fundamental Problem of Causal Inference – it is impossible to directly observe causal effects.