Beyond reasonable doubt

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The criminal standard of proof under English law:

“formerly described as “beyond reasonable doubt”. That standard remains, and the words commonly used, though the Judicial Studies Board guidance is that juries might be assisted by being told that to convict they must be persuaded “so that you are sure”

—Legal Studies Board guidance

“Are you trying to say you can’t convict anyone without direct evidence?”

“No, but I am saying it should be a lot harder, because without direct evidence you are dependent on probabilities.”

—Some wag on Twitter

Circumstantial evidence, beyond reasonable doubt, and the perfect crime

Circumstantial evidence as “information that changes the prior probability.”

It may seem outrageous that it is possible to commit a “perfect crime”, but as a general proposition under the common law one cannot be convicted of a crime for which there is no direct or indirect evidence.

This is a matter of ontology, even metaphysics, sheeted in epistemological scepticism. Here is Berkeley:

“We have no other notion of cause and effect, but that of certain objects, which have been always conjoined together, and which in all past instances have been found inseparable. We cannot penetrate into the reason of the conjunction. We only observe the thing itself, and always find that from the constant conjunction the objects acquire a union in the imagination.”

—David Hume

Ther’s almost a bit of Berkeley in this.

When we do our utmost to conceive the existence of external bodies, we are all the while only contemplating our own ideas. But the mind, taking no notice of itself, is deluded to think it can and does conceive bodies existing unthought of or without the mind; though at the same time they are apprehended by, or exist in, itself.

At some level, if we have no independent grounds for believing that Colonel Mustard murdered Reverend Green then, in our epistemology, Colonel Mustard did not murder Reverend Green even if, unbeknownst to us and unreflected in the available evidence, Colonel Mustard in fact did murder Reverend Green. We have no justified belief in that proposition.

But we do not need much information to form some kind of justified belief in it.

Direct v circumstantial

Those grounds fall into two categories: those that directly confirm or contradict it — which may take the form of an eye witness report that “Colonel Mustard murdered Reverend Green in the library with a lead pipe”, recorded video and audio of him doing so, Colonel Mustard’s confessions and so on which, if accepted, settle the matter — we call this “direct evidence” — and information that changes the probability of the proposition being true — Colonel Mustard’s alibi, a till receipt for a lead pipe, a long-standing feud between the two. This we call circumstantial evidence.

You might say that direct evidence is evidence which, if accepted, establishes causation while circumstantial evidence is evidence which, if accepted, establishes correlation. One confirms that the event actually happened, the other that it is more likely to have happened.

Direct evidence is in a way, a subset of circumstantial evidence in that it changes the probability of the event to 100% (or zero), and therefore the only question that remains is “how reliable is the evidence?” Was the witness mistaken, or lying? Was the confession coerced? Doubt about reliability may change its weighting to less than 100%

Circumstantial evidence requires the same assessment for reliability, but before that there is a question: even if completely reliable, how much does the evidence change the probability of the event in the first place. Colonel Mustard might be a plumber, so habitually in possession of lead piping. He may have bought the lead pipe, but then been robbed of it. Miss Scarlett, Mrs White, Mrs Peacock and Professor Plum may each have had access to lead pipe too. There may have been an unseen intruder with a lead pipe.

See also

• Miscarriages of justice