Circumstantial evidence is evidence in which an inference is required to connect it to a conclusion of fact, like a fingerprint at the scene of a crime. By contrast, direct evidence supports the truth of an assertion directly—i.e., without need for any additional evidence or the intervening inference.
On its own, it is the nature of circumstantial evidence for more than one explanation to still be possible. Inference from one piece of circumstantial evidence may not guarantee accuracy. Circumstantial evidence usually accumulates into a collection, so that the pieces then become corroborating evidence. Together, they may more strongly support one particular inference over another. An explanation involving circumstantial evidence becomes more valid as proof of a fact when the alternative explanations have been ruled out.
Circumstantial evidence allows a trier of fact to deduce a fact exists. In criminal law, the inference is made by the trier of facts in order to support the truth of assertion (of guilt or absence of guilt).And a definition from another on-line dictionary:
Circumstantial Evidence: Evidence not bearing directly on the fact in dispute but on various attendant circumstances from which the judge or jury might infer the occurrence of the fact in dispute.An Anthrax Truther arguing on this blog persists in arguing that the fact that the hidden message in the anthax letters sent to the media decodes to "PAT" doesn't mean anything, because there are countless people named "Pat" in the world. It doesn't have to pertain to Ivins' co-worker Pat Fellows.
No one disagrees that "Pat" doesn't have to pertain to Pat Fellows. As stated in the explanation of "circumstantial evidence" above, "it is the nature of circumstantial evidence for more than one explanation to still be possible."
However, in court, it would be explained to the jury that (1) Bruce Ivins had sent other types of "coded messages" to Pat Fellows; (2) that Ivins was somewhat obsessed with his female co-workers, including Pat Fellows; (3) that the other way to decode the message produced "FNY," which appears to pertain to friendly arguments Ivins had with his other female co-worker, Mara Linscott, who was a fan of the New York Yankees, while Ivins disliked the Yankees; (4) and that Ivins once loaned a copy of the book Godel, Escher, Bach to Pat Fellows, and that book contained the key to decoding the hidden message Ivins had put in the media letter.
So, it would be explained to the jury that it is very likely that the decoded name "Pat" refers to Ivins' co-worker Pat Fellows. It is a more likely explanation than any other explanation.
The Anthrax Truther will probably argue that being "very likely" isn't good enough. It must be a certainty. In reality, it does NOT have to be a certainty. It just has to be a logical way to decode the hidden message. The jury will decide if it is "very likely" that was what Bruce Ivins intended the coded message to mean.
No one disputes that, by itself, the name "Pat" doesn't mean anything. But when explained to the jury as part of a mass of evidence showing that Bruce Ivins was the anthrax killer, the jury will decide for itself if the explanation is logical and if it helps to convince them that Bruce Ivins was the anthrax killer.