In an ordinary copyright infringement case, say involving two literary works, what do we tell juries about how they are to find liability or not? I don't mean abstract principles like, "You have to find plaintiff's work is copyrightable, plaintiff owned the rights in question at the time defendant allegedly infringed them, that defendant acted without authorization, and that defendant copied a material amount of expression." I mean the nitty gritty of what we tell the jury that will be of actual assistance in finding that the two works are substantially similar in expression.
The lawyers will point to the testimony of witnesses and to analyses of the works themselves, maybe even to demonstrative charts with side-by-side comparisons from the works, but how do you assist the jury in deciding too much or too little of something was copied? "Substantial" includes both quantitative and qualification aspects on top of other problems.
In Arnstein v. Porter, 154 F.2d 464 (2d Cir. 1946), the court of appeals phrased matters this way: "The question, therefore, is whether defendant took from plaintiff's works so much of what is pleasing to the ears of lay listeners, who comprise the audience for whom such popular music is composed, that defendant wrongfully appropriated something which belongs to the plaintiff." This is of course, completely circular: how does the jury know when something belongs to plaintiff in the first place and if so to such an extent that defendant should pay in damages?