In a criminal trial, the prosecution must not only show that the defendant committed the crime in question, but that he or she also had the necessary intent or mental state to commit that crime. The prosecution may prove intent or mental state using direct evidence, circumstantial evidence, or a combination of both.
All evidence may be classified as either direct evidence or indirect evidence. Indirect evidence is also referred to as circumstantial evidence. Indirect or circumstantial evidence is distinguished from direct evidence in that it does not point to the fact in question, but rather to a fact or set of facts from which the fact in question may be inferred.
To illustrate this difference, consider a witness who watches John shoot Mike and kill him. The witness’s testimony would be considered direct evidence against John. On the other hand, a witness who testifies that he watched John pick up a gun and follow Mike into another room, then heard a gunshot and saw John walk out of the room holding a smoking gun would only be giving indirect, or circumstantial, evidence against John. In this case, it would be up to the prosecution to convince the jury that the facts of the case reasonably lead to the conclusion that John shot Mike.
If a defense attorney or prosecutor introduces circumstantial evidence into the trial, it must be shown that the evidence only supports one reasonable conclusion before the jury may make a decision based upon that evidence. For example, if the circumstantial evidence leads to two reasonable conclusions—one that the defendant had the required intent or mental state and the other that the defendant did not—then the jury must conclude that the defendant did not have the intent or mental state to commit the crime.