What does but for rule mean?

but for rule meaning in Legal Dictionary

n. one of the examinations to find out if a defendant accounts for a certain happening. Within test, had been indeed there other cause, or would it not have taken place "but also for" the defendant's actions? Instance: "But for" defendant Drivewild's speeding, the automobile will never have gone out of hand, and then the defendant is accountable. It is shorthand for whether or not the activity was the "proximate cause" of damage.