McDowell v. Davis
In McDowell v. Davis, 104 Ariz. 69, 71-72, 448 P.2d 869, 871-72 (1968), the Arizona Supreme Court disapproved a jury instruction informing jurors that proximate cause is demonstrated if the negligent act was a substantial factor, rather than a slight or possible factor, in producing the injury.
The court wrote that if it could be assured that jurors understood the term "substantial factor" to mean not imaginary, illusive or insignificant, the court would not dispute its use. Id. at 71, 448 P.2d at 871.
The court reasoned, however, that because the word "substantial" commonly refers to a large quantity, the instruction implied that a tortfeasor's act or omission must be a "large" cause of a plaintiff's damages. Id. at 71-72, 448 P.2d at 871-72.
In fact, because a tortfeasor can be liable if its conduct contributed "only a little" to the plaintiff's damages, the court decided that use of the word in a jury instruction would be misleading. Id.
The court then reiterated the oft-stated rule that proximate cause consists of "any cause which in a natural and continuous sequence produces the injury and without which the result would not have occurred." Id.