Reading the constitutional and statutory definitions of patentable subject matter, it would seem that you could patent almost anything. The Constitution gives Congress the power to grant “inventors the exclusive right to their … discoveries.” The most recent provision by Congress on patentable subject matter, 35 U.S.C. §101, enacted in 1952, allows a patent to be obtained on “any new and useful process, machine, manufacture, or composition of matter, or any new and useful improvement thereof.”  

Despite that broad language, there are limits on what types of subject matter can be patented. From early on, judicial “exceptions” have narrowed the scope of patentable subject matter, excluding things such as abstract ideas and laws of nature. See, e.g., Le Roy v. Tatham, 55 U.S. (14 How.) 156, 175 (1852) (“A principle, in the abstract, is a fundamental truth; an original cause; a motive; these cannot be patented, as no one can claim in either of them an exclusive right.”). Nevertheless, for a long time, the exception remained in the background and did not seem to pose a threat to patents in most areas of technology.