When does the "substantial factor" test apply?

The "substantial factor" test is primarily applied in situations where multiple causes contribute to the same injury. This test is used to determine if a defendant's conduct was a substantial factor in bringing about the injury when several different factors may have played a role.

In cases involving multiple tortfeasors, the courts recognize that a single injury can often result from the combined actions of multiple parties, each potentially contributing to the harm. The "substantial factor" test provides a framework to establish liability even when it is difficult to apportion exact blame among the various causes. It acknowledges that while one party may not have been the sole cause, their actions were still a significant factor in causing the harm suffered by the plaintiff.

The other options focus on scenarios that do not require the use of the "substantial factor" test. For instance, if there is clear evidence of a defendant's negligence, traditional causation rules may be more straightforwardly applied without needing this test. Similarly, when an injury occurs without any external causes or when a plaintiff cannot prove any causation, the substantial factor standard is not necessary or relevant as there are other legal doctrines that govern those circumstances.