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"