Some very basic background from a famous patent case (Diamond v. Chakrabarty):

"Anything under the sun that is made by man [is patentable] . . . . [T]he laws of nature, physical phenomena, and abstract ideas have been held not patentable . . . a new mineral discovered in the earth or a new plant found in the wild is not patentable subject matter. Likewise, Einstein could not patent his celebrated law that E=mc2; nor could Newton have patented the law of gravity. Such discoveries are manifestations of nature, free to all men and reserved exclusively to none."