"511 Restricting Innovation, 56 Stan. L.Rev. 1345, 1386-1390 (2004). The argument for imposing indirect liability in this case is, however, a powerful one, given the number of infringing downloads that occur every day using StreamCast’s and Grokster’s software. When a widely shared service or product is used to commit infringement, it may be impossible to enforce rights in the protected work effectively against all direct infringers, the only practical alternative being to go against the distributor of the copying device for secondary liability on a theory of contributory or vicarious infringement. One infringes contributorily by intentionally inducing or encouraging direct infringement, see Gershwin Pub. Corp. v. Columbia Artists Management, Inc., 443 F.2d 1159, 1162 (C.A.2 1971), and infringes vicariously by profiting from direct infringement while declining to exercise a right to stop or limit it, Shapiro, Bernstein & Co. v. H.L. Green Co., 316 F.2d 304, 307 (C.A.2 1963). 4 Although “[t]he Copyright Act does not expressly render anyone liable for infringement committed by another,” Sony Corp. v. Universal City Studios, 464 U.S., at 434, these doctrines of secondary liability emerged from common law principles and are well established in the law, id., at 486 (Blackmun, J., dissenting). 4 We stated in Sony Corp. of America v. Universal City Studios, Inc., 464 U.S. 417 (1984), that “‘the lines between direct infringement, contributory infringement and vicarious liability are not clearly drawn’….[R]easoned analysis of [the Sony plaintiffs’ contributory infringement claim] n ecessarily entails consideration of arguments and case law which may also be forwarded under the other labels, and indeed the parties … rely upon such arguments and authority in support of their respective positions on the issue of contributory infringemen t,” id., at 435, n. 17. In the present case MGM has argued a vicarious liability theory, which allows imposition of liability when the defendant profits directly from the infringement and has a right and ability to supervise the direct infringer, even if the defendant initially lacks knowledge of the infringement. Because we resolve the case based on an inducement theory, there is no need to analyze separately MGM’s vicarious liability theory."