Indeed, proof that a given condition always precedes or accompanies a phenomenon does not warrant concluding with certainty that a given condition is the immediate cause of that phenomenon. It must still be established that when this condition is removed, the phenomenon will no longer appear.

Claude Bernard (1813–1878) French physiologist, scientist

An Introduction to the Study of Experimental Medicine [Introduction à l’Étude de la Médecine Expérimentale] (1865)