Parker v. Flook
In Parker v. Flook, 437 U.S. 584 (1978), the U.S. Supreme Court rejected a patent for a method for updating an "alarm limit" value on a variable (such as temperature) used for the a process of catalytic chemical conversion of hydrocarbons.
The Court held that the patent claim was merely one for a method consisting of an "algorithm or mathematical formula" along with "conventional post-solution applications" of the formula. The Court found that the applicant had failed to show that his claim did not "preempt the mathematical formula" merely because it was limited to one alleged field of use (the catalytic conversion of hydrocarbons). The Court held that the same defect rendered the subject matter non-patentable in Benson, where "there was a specific end use contemplated for the algorithm -- utilization of the algorithm in computer programming."
The Parker Court used the same definition set forth by Gottschalk v. Benson for the concept of an "algorithm", which is excluded from patentability. Parker v. Flook, 437 U.S. 584, 585 n.1 (1978).
Justice Potter Stewart dissented in Parker, pointing out that "a claimed process [does not lose] its status of subject-matter patentability simply because one step in the process would not be patentable subject matter if considered in isolation."