A more thorough explanation:

The but-for test is a legal test used in both tort law and criminal law to determine actual causation. It asks the question, "but for the existence of X, would Y have occurred?"

In tort law, but-for causation is a requirement for liability in combination with proximate cause. If either of these is absent, a party cannot be held liable.

However, the but-for test has some problems. For example, it can be difficult to determine the cause of an event when there are multiple contributing factors. Additionally, the test can be too broad, making it possible to attribute liability to parties who are only tangentially related to an event.

To address these issues, courts have developed various approaches. In tort law, proximate cause is required in addition to but-for causation. In criminal law, courts may use the substantial factor test, the likelihood of survival test, the acceleration theory, the doctrine of proximate cause, or the Model Penal Code approach.

For example, under the substantial factor test, if two people shoot someone at the same time, both gunmen would be found guilty despite ambiguity about whose bullet caused the death. However, this test can also lead to problems, such as holding members of a firing squad liable even if their bullets did not harm the victim.

Overall, the but-for test is an important tool in determining causation in legal cases, but it must be used carefully and in conjunction with other legal principles.