For an invention to be patentable, patent laws require that it be: patentable subject matter, that is, a kind of subject-matter that is eligible for patent protection. The official patentability examination of a patent application is performed by a patent examiner. Inventorship is also regarded as a patentability criterion. It is a constitutional requirement. Congress' ability to grant patents is authorized only for the inventor. Patentability refers to "substantive" conditions, not formal conditions such as the "sufficiency of disclosure", the "unity of invention" or the "best mode requirement." For an invention to be patentable, that is, to be eligible for a grant of patent, certain relevant legal conditions must be met. Patentability also refers to the substantive conditions that must be met for a patent to be held valid.
The grant of a patent creates a presumption that the claimed invention is patentable, though errors in the granting procedure may occur and previously unconsidered prior art may be brought to light only after the grant. The question whether there is a patentable invention, is as fugitive, impalpable, wayward, and vague a phantom as exists in the whole paraphernalia of legal concepts.
The test of invention is the originality of the discovery, and discovery depends upon the mental act of conceiving the new combination, for substantially every invention is only a combination.
"Inventorship is indeed relevant to patentability under 35 U.S.C. § 102(f), and patents have in the past been held unenforceable for failure to correctly name inventors in cases where the named inventors acted in bad faith or with deceptive intent." - Board of Education ex rel Board of Trustees of Florida State University v. American Bioscience Inc, 67 USPQ 2d 1252 (Fed Cir 2003).
Courts never tire, or at least in earlier times they never did, of expatiating upon the freshness of insight which observes a little, but fruitful, change which had theretofore escaped detection by those engaged in the field. - US Judge Learned Hand in Harries v. Air King Prod. Co., 183 F.2d 158, 162 (2d Cir. 1950).
A patentable invention may infringe another patent. The first patent in an area may have a broad claim covering the concept of the invention since there is no prior art in that particular area. A specific patentable implementation of that concept, not disclosed in the earlier patent but falling within the claim to the general concept, may be invented later.
Thomas Edison's thin carbon filament light bulb was a patentable improvement over the earlier patented Woodward and Evans thick carbon filament lightbulb. Patentability opinion from a patent agent or patent attorney as to whether an invention satisfies the substantive conditions of patentability is recommended.