In City of Oklahoma City v. Tuttle, 471 U.S. 808, 105 S.Ct. 2427, 2436, 85 L.Ed.2d 791 (1985), the Supreme Court set forth an inquiry for determining whether the necessary causal relationship between policy and constitutional deprivation is present in a given case: "Proof of a single incident of unconstitutional activity is not sufficient to impose liability under Monell, unless proof of the incident includes proof that it was caused by an existing, unconstitutional municipal policy, which policy can be attributable to a municipal policymaker. Otherwise the existence of the unconstitutional policy, and its origin, must be separately proved. But where the policy relied upon is not itself unconstitutional, considerably more proof than the single incident will be necessary in every case to establish both the requisite fault on the part of the municipality, and the causal connection between the "policy" and the constitutional deprivation." In City of Oklahoma City v. Tuttle, 471 U.S. 808, 105 S.Ct. 2427, 85 L.Ed.2d 791 (1985), where the majority overturned a judgment against a municipality because the charge allowed the jury to infer " 'gross negligence' or 'deliberate indifference' " from a non-policymaker's single excessive use of force, id. at 821, 105 S.Ct. at 2435, he concurred, and, writing for himself and Justices Marshall and Blackmun, observed that the plaintiff "bore the burden ... of proving" that his injuries resulted from " 'conscious choices' ... made by the city concerning police training and supervision," id. at 829-30 n. 4, 105 S.Ct. at 2439 n. 4. In Tuttle, moreover, Justice Brennan explicitly rejected mere "but for" causality, invoking at least the limiting principles of tort law. Tuttle, 471 U.S. at 833 n. 8, 105 S.Ct. at 2441 n. 8.