""If x is a sufficient cause of y, then the presence of x necessarily implies the presence of y. However, another cause z may alternatively cause y. Thus the presence of y does not imply the presence of x.""
Wikipedia contributors, ""Causality,"" Wikipedia, The Free Encyclopedia, http://en.wikipedia.org/w/index.php?title=Causality&oldid=88995105 (accessed November 27, 2006).