The classic paper The Environment and Disease: Association or Causation? (Proc. Royal Society of Medicine, May 1965, vol. 58, no. 5, pp. 295–300) deals with an ever present scientific problem: Having established or observed that two variables are associated, on what grounds can we reasonably infer that one causes the other? The author is Sir Austin Bradford Hill, co-author of the first paper to establish a link between smoking and lung cancer.