See also: Proximate cause, Causation

In tort law, there are three essential elements to every cause of action: (1) existence of a duty, (2) breach of the duty by the defendant, (3) causation of harm.

Sine qua non1 or conditio sine qua non is one theory of causation. Basically, the defendant's breach of duty (element (2)) will be considered the conditio sine qua non of the plaintiff's injury if "but for" defendant's act or omission, the harm would not have been sustained. This is what is known as the "cause in fact."

However, sine qua non can be both overinclusive and underinclusive. For example, if A fires a starter pistol, which causes birds in the next county to scatter about madly, eventually running into a small aircraft and causing it to crash, clearly, A's firing of the starter pistol is the conditio sine qua non - if he hadn't spooked the birds by firing the pistol, the birds wouldn't have caused the plane to wreck. However, it does not necessarily seem reasonable to hold people responsible for the remote and unforeseeable consequences of their actions. Thus, in U.S. tort law, the line between causation and dutyis often hazy. This rather poorly delineated blob is known as "legal causation." In order to satisfy the causation requirement, the defendant's wrongful act must not only be the "cause in fact" (conditio sine qua non), but also the legal, "proximate cause."

1 Lat. "without which, not"