In Sony Corp. v. Universal City Studios, Inc., 464 U.S. 417 (1984), the Court decided that Sony's manufacturing and selling of VTR's did not infringe the copyrights of Universal Studios and Walt Disney. Recording TV broadcasts through the device for later viewing (a.k.a. "time-shifting") was considered a "fair use."
An excerpt from the Court opinion says,
When these factors are all weighed in the "equitable rule of reason" balance, we must conclude that this record amply supports the District Court's conclusion that home time-shifting is fair use.
My question is, what is the exact definition of this "equitable rule of reason" and what is the legal basis for this doctrine?
Furthermore, how is this applied in Sony Corp. v. Universal City Studios, Inc. or other fair use cases?