Actus reus, which is shorthand for the opening words in the Latin phrase actus non facit reum nisi mens sit rea (“an act does not make a person guilty unless his mind is also guilty”), is one element of a crime that a prosecutor must prove to establish criminal liability. A prosecutor must prove, in particular, that the defendant’s actus reus caused the harmful result at issue in the case. To do so, the prosecutor must show not only that the act was the “actual cause” of the harm (i.e., the “factual cause” or the “but for” cause”) but also that the act was the “proximate cause” of the harm (i.e., the “legal cause”).

The so-called “but for” test, also known as the sine qua non test, seeks to determine whether a particular act brought about the particular harm to the alleged victim. If the question whether the harm would not have happened but for the defendant’s action is answered in the affirmative, then causation is established; accordingly, if the harm would have happened notwithstanding the defendant’s act, then the defendant’s act is not a “cause in fact.” The “but for” test is not satisfied unless the prosecutor can show that the harm was foreseeable; if the harm was not foreseeable, then the defendant cannot be said to be the actual cause of the harm, only the proximate cause of the harm.

Determining causation is difficult when two people are performing different acts at different times, and each of their acts could have caused the harm at the time the harm occurred. The two acts by the two different people constitute concurrent sufficient causes under the “but for” test. Because there are two different people who could have “caused” the harm according to the “but for” test, yet only one of the two people actually caused the harm, the “but for” test fails to establish causation.

There are two tests that courts may apply when there are multiple sufficient causes under the facts. The first is the substantial factor test, according to which a defendant is criminally liable if his acts are shown to be a substantial factor leading to the harm to the alleged victim. This test is not commonly used because it can be arbitrary and subjective. The better test is a modified form of the “but for” test, formulated this way: “But for the defendant’s voluntary act, the harm would not have occurred not just when it did, but as it did.” Even this revised test falls short of ideal. For instance, it is not clear how this test is applied when two non-lethal acts combine to cause the death of one victim.

Regardless of which tests for causation obtain or prevail in a particular case, a prosecutor must establish each element of a crime beyond a reasonable doubt. That standard, at least, is a legal certainty.