Counterfactual

A counterfactual is the outcome that would have happened under a scenario that did not actually occur — for example, how a patient who took a drug would have fared if they had not taken it. Causal effects are differences between what happened and this imagined alternative.

Counterfactuals are powerful but unobservable: each person either takes the drug or does not, so we never see both versions of the same person. This is sometimes called the fundamental problem of causal inference, and every causal method is essentially a clever way to estimate the missing counterfactual.