Perpetual motion machine in the context of "Utility (patent)"

In United States patent law, utility is a patentability requirement. As provided by 35 U.S.C. § 101, an invention is "useful" if it provides some identifiable benefit and is capable of use and "useless" otherwise. The majority of inventions are usually not challenged as lacking utility, but the doctrine prevents the patenting of fantastic or hypothetical devices such as perpetual motion machines.

The patent examiners guidelines require that a patent application expresses a specific, credible, and substantial utility. Rejection by an examiner usually requires documentary evidence establishing a prima facie showing that there is no specific, substantial, and credible utility.