In an article on “Contributory Negligence” in the January number of the Law Quarterly Review, the learned writer, Lord Justice O'Connor, draws attention to the state of confusion which a study of recent cases reveals as reigning with regard to this subject, and concludes by suggesting that the bare question should be put to the jury, “Whose fault was it?”. This article is not intended as a criticism of that thesis. But if the condition of principle is such that experience suggests its total abandonment, it is surely high time that Theory bestirred herself. The following observations constitute an attempt to apply the current formula to a particular class of cases—those namely which may be described as collision and running down cases—with the intention of showing that the formula does adequately cover the cases.