Pro-Sys Consultants Ltd. v. Microsoft Corporation, 2013 SCC 57 holds that indirect purchasers have a cause of action against the party who has effectuated the overcharge at the top of the distribution chain that has allegedly injured the indirect purchasers as a result of the overcharge being "passed on" to them through the chain of distribution. The argument that indirect purchasers should have no cause of action because passing on has been rejected as a defence in Canada should fail. The Court holds:
 The passing-on defence was typically advanced by an overcharger at the top of a distribution chain. It was invoked under the proposition that if the direct purchaser who sustained the original overcharge then passed that overcharge on to its own customers, the gain conferred on the overcharger was not at the expense of the direct purchaser because the direct purchaser suffered no loss. As such, the fact that the overcharge was "passed on" was argued to be a defence to actions brought by the direct purchaser against the party responsible for the overcharge.
 The passing-on defence has been rejected in both Canadian and U.S. jurisprudence. It was first addressed by the Supreme Court of the United States in 1968 in Hanover Shoe, Inc. v. United Shoe Machinery Corp., 392 U.S. 481 (1968). In that case, Hanover sued United for damages under U.S. antitrust laws because United would only lease, not sell, its shoe machinery, which Hanover claimed resulted in an overcharge to it. United argued that Hanover had passed on the overcharge to its own customers and had therefore suffered no harm. The U.S. Supreme Court (per White J., Stewart J. dissenting) rejected the passing-on defence to overcharging. It cited difficulties in ascertaining the nature and extent of the passing on of the overcharge as the reason for rejecting the defence:
Even if it could be shown that the buyer raised his price in response to, and in the amount of, the overcharge and that his margin of profit and total sales had not thereafter declined, there would remain the nearly insuperable difficulty of demonstrating that the particular plaintiff could not or would not have raised his prices absent the overcharge or maintained the higher price had the overcharge been discontinued. Since establishing the applicability of the passing-on defense would require a convincing showing of each of these virtually unascertainable figures, the task would normally prove insurmountable. [p. 493]
 The court added that to leave the only actionable causes in the hands of the indirect purchasers who "have only a tiny stake in a lawsuit and little interest in attempting a class action", would mean that "those who violate the antitrust laws by price fixing or monopolizing would retain the fruits of their illegality" (Hanover Shoe, at p. 494). The court thus rejected the passing-on defence. Since Hanover Shoe, defendants who effectuate illegal overcharges have been precluded from employing the passing-on defence as a means of absolving themselves of liability to their direct purchasers.
 The passing-on defence was rejected in Canada in Kingstreet Investments Ltd. v. New Brunswick (Finance), 2007 SCC 1,  1 S.C.R. 3, in the context of a claim for the recovery of taxes paid pursuant to ultra vires legislation. The dispute in that case arose out of a claim for the recovery of ultra vires user charges on liquor levied by the province of New Brunswick against Kingstreet Investments, whose business, among other things, involved the operation of night clubs. Bastarache J., writing for a unanimous Court, held that a public authority who had illegally overcharged a taxpayer could not reduce its liability for the overcharge simply by establishing that some or all of the overcharge was passed on to the taxpayer's customers.
 Bastarache J. found the passing-on defence to be inconsistent with the basic premise of restitution law. Basic restitutionary principles "provide for restoration of 'what has been taken or received from the plaintiff without justification'. . . . Restitution law is not concerned by the possibility of the plaintiff obtaining a windfall precisely because it is not founded on the concept of compensation for loss" (Kingstreet, at para. 47, quoting Commissioner of State Revenue (Victoria) v. Royal Insurance Australia Ltd. (1994), 182 C.L.R. 51 (H.C.A.) at p. 71). Accordingly, "[a]s between the taxpayer and the Crown, the question of whether the taxpayer has been able to recoup its loss from some other source is simply irrelevant" (Kingstreet, at para. 45, quoting P. D. Maddaugh and J. D. McCamus, The Law of Restitution (loose-leaf updated September 2005), at p. 11-45).
 Bastarache J. also found the passing-on defence to be "economically misconceived" (Kingstreet, at para. 48). By this he accepted that the task of determining the ultimate location of the harm of the overcharge is "exceedingly difficult and constitutes an inappropriate basis for denying relief" (para. 44). Echoing the misgivings expressed in Hanover Shoe, he cited the inherent difficulty in accounting for the effects of market elasticities on the prices charged by direct purchasers as the basis for this conclusion. He found these complexities made it impossible to tell what part, if any, of the overcharge was actually passed on (Kingstreet, at para. 48).
 Pro-Sys says that Kingstreet stands only for the rejection of the defence in the context of ultra vires taxes. In my view, however, there are three reasons that lead to the conclusion that Bastarache J.'s rejection of the passing-on defence in Kingstreet was not limited to that context.
 First, this Court's jurisprudence supports the broader rejection of the passing-on defence. In British Columbia v. Canadian Forest Products Ltd., 2004 SCC 38,  2 S.C.R. 74 ("Canfor"), the Crown claimed "diminution of the value of the timber" that it sold, following a forest fire caused largely by Canfor. Though the Court ultimately held in that case that the Crown had not in fact suffered loss because it was able to recover its damages through the regulatory scheme it had instituted, Binnie J. stated (albeit in obiter) that "[i]t is not generally open to a wrongdoer to dispute the existence of a loss on the basis it has been 'passed on' by the plaintiff" because this would burden courts with "the endlessness and futility of the effort to follow every transaction to its ultimate result" (para. 111, quoting Southern Pacific Co. v. Darnell-Taenzer Lumber Co., 245 U.S. 531 (1918), at p. 534). Likewise, in the same decision LeBel J., dissenting, though not on this point, said that "the passing-on defence, on the facts of this case and generally, must not be allowed to take hold in Canadian jurisprudence" (para. 197). To allow otherwise, LeBel J. indicated, would force a difficult burden of proof on the plaintiff to demonstrate not only that it had suffered a loss, but that it did not engage in any other transactions that would have offset the loss (para. 203).
 In Kingstreet, Bastarache J. endorsed the reasons for rejecting the passing-on defence advanced by LeBel J. in the tort law context in Canfor, saying such rejection was of equal if not greater consequence in restitution law (para. 49).
 Second, in Kingstreet, Bastarache J. found that the rejection of the passing-on defence was consistent with basic restitutionary law principles. Specifically, the rejection of the defence accords with the principle against unjust enrichment or nullus commodum capere potest de injuria sua propria (barring wrongdoers from benefiting from their unlawful actions). Preventing defendants from invoking passing on as a defence helps to ensure that wrongdoers are not permitted to retain their ill-gotten gains simply because it would be difficult to ascertain the precise extent of the harm. Likewise, it is important as a matter of restitutionary law to ensure that wrongdoers who overcharge their purchasers do not operate with impunity, on the grounds that complexities in tracing the overcharge through the chain of distribution will serve to shield them from liability.
 Finally, there is support in the academic commentary for the broader rejection of the passing-on defence. Maddaugh and McCamus have stated that Kingstreet was an "authoritative and apparently comprehensive rejection" of the passing-on defence in Canada, and that "[i]n reaching this conclusion, the Supreme Court reflected a broad international consensus with respect to the unsuitability of this defence" (p. 11-46 of 2013 update).
 For these reasons, I conclude that the rejection of the passing-on defence in Kingstreet is not limited to the context of the imposition of ultra vires taxes. There is no principled reason to reject the defence in one context but not another; the passing-on defence is rejected throughout the whole of restitutionary law.