Circumstantial evidence
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Crime & Punishment
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Circumstantial evidence
(n.)
Evidence that changes the prior probability of a fact in issue in legal proceedings without directly attesting to it.”
“Direct” v “circumstantial” evidence
Circumstantial evidence is to be distinguished from direct evidence, which addresses the causal proposition implied in a crime directly.
Let us say the allegation is “JC competed in the Tour de France”. (Let us at once remark what a scurrilous allegation this would be were we not in the realm of the fantastic hypothetical)
“Birgit von Sachsen Rampton saw JC competing in the Tour de France on the television” would be direct evidence of the allegation.
“JC just bought a ludicrously expensive bike, regularly bedecks himself in lycra, talks a lot about Bradley Wiggins and has a one-way Eurostar ticket to Paris for the weekend of 29 June” is — rather weak, it must be said — circumstantial evidence of the allegation.
The distinction is important in criminal law. So let us talk about a hypothetical crime.
Colonel Mustard murdered Reverend Green.
Direct evidence may take the form of an eye witness report from Mrs. Peacock that, say, “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. If we believe the witness, this evidence will settle the matter.
Circumstantial evidence is information, if we accept it, changes the probability of the allegation being true. Colonel Mustard’s alibi that he was playing canasta with Mrs White all evening. A till receipt in his wallet for lead pipe. Evidence of a long-standing feud between Mustard and Green.
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 resets the probability of the event either to 100% or to zero. The only remaining question is “how reliable is the evidence?” Was the witness mistaken? Was she lying? Doubt about reliability may change its weighting to less than 100%
Circumstantial evidence requires the same assessment for reliability, but before even 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 indeed 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.