 A fifth modification to the counterfactual test of cause in fact is more by way of substitution than of amendment. This is the First and Second Restatement of Torts “substantial factor” test. Motivated mostly by worries about overdetermination cases, the American Law Institute in both of its first two Restatements urged that a “substantial factor” test be substituted for sine qua non as the test for cause-in-fact in torts. The test asks only whether a defendant’s action was a substantial factor in the production of the harm complained of. This admittedly circular and vague test was thought to help in overdetermination cases like that of the joint fires, because so long as each fire was quite substantial (in comparison to the other fire) each was a cause of the harm, even though neither fire was a necessary condition of the harm.