Knote v. United States was a United States Supreme Court case that dealt with the issue of whether a patent for a new invention could be granted if the invention was already in use by another person. The case involved a patent application by John Knote for a new type of steam engine. Knote argued that he was the first to invent the engine, and that he should be granted a patent for it. The United States argued that the engine was already in use by another person, and thus Knote was not entitled to a patent. The Supreme Court ultimately sided with the United States, ruling that a patent could not be granted for an invention that was already in use by another person. The Court reasoned that the patent system was designed to reward inventors for their original work, and that granting a patent for an invention that was already in use would be contrary to this purpose. The Court also noted that the patent system was intended to encourage innovation, and that granting a patent for an invention that was already in use would discourage innovation. The Court's ruling in Knote v. United States established the principle that a patent cannot be granted for an invention that is already in use by another person. This principle has been applied in numerous subsequent cases, and remains an important part of patent law today.

Justice Field delivered the dissenting opinion in Knote v. United States, arguing that the majority's decision was contrary to established precedent and would have far-reaching implications for government contracts. He argued that under existing law, a contract between two parties could not be modified without their mutual consent; thus, he asserted that Congress had no authority to modify an existing contract with the plaintiff by passing a statute which changed its terms. Furthermore, Justice Field argued that if Congress were allowed to do so in this case it would set a dangerous precedent whereby any party who entered into an agreement with the federal government could find themselves subject to unilateral changes at any time. Finally, Justice Field noted his concern about how such legislation might be used as leverage against citizens or corporations by future administrations seeking political gain or other ends.