What is the test for determining whether a defendant's reasonableness of their conduct is reasonable?

This court in the past has repeatedly recognized that judging the reasonableness of a defendant's conduct on a case-by-case basis provides a more precise determination of the contours of liability: "Standards of care are typically relative; rules of law are basically absolute. Hence, in regard to negligence, any attempt to screen factual conduct into legal classifications through a sieve of absolute law will be impracticable whenever the related circumstances admit of materially conflicting inferences. In other words, the actor's conduct must always be gauged in relation to all the other material circumstances surrounding it...." (Toschi v. Christian, supra, 24 Cal.2d 354, 360, 149 P.2d 848.)