A decision of the House of Lords (now the Supreme Court) on cases where it may be difficult to identify what caused an incident. Lord Brandon said,

My Lords, the appeal does not raise any question of law, except possibly the question what is meant by proof of a case “on a balance of probabilities.” Nor do underwriters challenge before your Lordships any of the primary findings of fact made by Bingham J.

The question, and the sole question, which your Lordships have to decide is whether, on the basis of those primary findings of fact, Bingham J. and the Court of Appeal were justified in drawing the inference that the ship was, on a balance of probabilities, lost by perils of the sea.

In approaching this question it is important that two matters should be borne constantly in mind. The first matter is that the burden of proving, on a balance of probabilities, that the ship was lost by perils of the sea, is and remains throughout on the shipowners. Although it is open to underwriters to suggest and seek to prove some other cause of loss, against which the ship was not insured, there is no obligation on them to do so. Moreover, if they chose to do so, there is no obligation on them to prove, even on a balance of probabilities, the truth of their alternative case.

Lord Brandon also said,

My Lords, the late Sir Arthur Conan Doyle in his book “The Sign of Four” , describes his hero, Mr. Sherlock Holmes, as saying to the latter’s friend, Dr. Watson: “how often have I said to you that, when you have eliminated the impossible, whatever remains, however improbable, must be the truth?” It is, no doubt, on the basis of this well-known but unjudicial dictum that Bingham J. decided to accept the shipowners’ submarine theory, even though he regarded it, for seven cogent reasons, as extremely improbable.

In my view there are three reasons why it is inappropriate to apply the dictum of Mr. Sherlock Holmes, to which I have just referred, to the process of fact-finding which a judge of first instance has to perform at the conclusion of a case of the kind here concerned.

The first reason is one which I have already sought to emphasise as being of great importance, namely, that the judge is not bound always to make a finding one way or the other with regard to the facts averred by the parties. He has open to him the third alternative of saying that the party on whom the burden of proof lies in relation to any averment made by him has failed to discharge that burden. No judge likes to decide cases on burden of proof if he can legitimately avoid having to do so.

There are cases, however, in which, owing to the unsatisfactory state of the evidence or otherwise, deciding on the burden of proof is the only just course for him to take.

The second reason is that the dictum can only apply when all relevant facts are known, so that all possible explanations, except a single extremely improbable one, can properly be eliminated. That state of affairs does not exist in the present case: to take but one example, the ship sank in such deep water that a diver’s examination of the nature of the aperture, which might well have thrown light on its cause, could not be carried out.

The third reason is that the legal concept of proof of a case on a balance of probabilities must be applied with common sense. It requires a judge of first instance, before he finds that a particular event occurred, to be satisfied on the evidence that it is more likely to have occurred than not. If such a judge concludes, on a whole series of cogent grounds, that the occurrence of an event is extremely improbable, a finding by him that it is nevertheless more likely to have occurred than not, does not accord with common sense. This is especially so when it is open to the judge to say simply that the evidence leaves him in doubt whether the event occurred or not, and that the party on whom the burden of proving that the event occurred lies has therefore failed to discharge such burden.

In my opinion Bingham J. adopted an erroneous approach to this case by regarding himself as compelled to choose between two theories, both of which he regarded as extremely improbable, or one of which he regarded as extremely improbable and the other of which he regarded as virtually impossible. He should have borne in mind, and considered carefully in his judgment, the third alternative which was open to him, namely, that the evidence left him in doubt as to the cause of the aperture in the ship’s hull, and that, in these circumstances, the shipowners had failed to discharge the burden of proof which was on them.

This case was reviewed in O’Connor.