CACI No. 430 reads as follows: "A substantial factor in causing harm is a factor that a reasonable person would consider to have contributed to the harm. It must be more than a remote or trivial factor. It does not have to be the only cause of the harm. Conduct is not a substantial factor in causing harm if the same harm would have occurred without that conduct." (6 Witkin, Summary of Cal. Law, supra, (2010 supp.) 1187, p. 65.) CACI No. 431 provides: "A person's negligence may combine with another factor to cause harm. If you find that name of defendant's negligence was a substantial factor in causing name of plaintiff's harm, then name of defendant is responsible for the harm. Name of defendant cannot avoid responsibility just because some other person, condition, or event was also a substantial factor in causing name of plaintiff's harm." (6 Witkin, Summary of Cal. Law, supra, 1187, pp. 554-557.) In Mitchell v. Gonzales (1991) 54 Cal.3d 1041, the court identified several problems that are solved by substantial factor analysis, in assigning different levels of fault to different types of contributions to injury: "One is where a similar, but not identical result would have followed without the defendant's act; the other where one defendant has made a clearly proved but quite insignificant contribution to the result, as where he throws a lighted match into a forest fire." (Id. at pp. 1052-1053.) "Causation in the law of negligence is not determined by a linear projection from a 'but for' premise. Instead, it is expressed in terms of 'foreseeability' and is limited by the policy that cause must be 'proximate.' the problem is complex, and has bedeviled many." (Brewer v. Teano (1995) 40 Cal.App.4th 1024, 1030.) Several issues arise on foreseeability. "In some contexts it is a question of fact for the jury, and in others a 'part of the calculus to which a court looks in defining the boundaries of "duty."' . . . the jury considers foreseeability in two more focused, fact-specific settings: likelihood or foreseeability of injury in deciding whether defendant's conduct was negligent in the first place, and whether the negligence was a proximate or legal cause of plaintiff's injury." (Ibid.; ) No particular degree of contribution to harm is required for liability establishment: "'The substantial factor standard is a relatively broad one, requiring only that the contribution of the individual cause be more than negligible or theoretical.' Thus, 'a force which plays only an "infinitesimal" or "theoretical" part in bringing about injury, damage, or loss is not a substantial factor' , but a very minor force that does cause harm is a substantial factor . This rule honors the principle of comparative fault.'" (Bockrath v. Aldrich Chemical Co., Inc. (1999) 21 Cal.4th 71, 79.)