Just because an invention falls within one of the categories of patentable subjects, it is not necessarily patentable. The Patent Act and judicial interpretations have established certain tests that must first be met. To approve a patent application, the PTO (as part of the Department of Commerce) will require that the invention, discovery, or process be novel, useful, and nonobvious in light of current technology.
Perhaps the most significant test of patentability is that of obviousness. The act says that no invention may be patented “if the differences between the subject matter sought to be patented and the prior art are such that the subject matter as a whole would have been obvious at the time the invention was made to a person having ordinary skill in the art to which said subject matter pertains.” This provision of the law has produced innumerable court cases, especially over improvement patents, when those who wish to use an invention on which a patent has been issued have refused to pay royalties on the grounds that the invention was obvious to anyone who looked.