The reach of the patent system has substantially broadened in recent decades. Subject matter extensions were not introduced by parliaments, but by individual judges considering specific cases, often between private parties. The focus in this thesis is whether these changes create a net economic benefit to society. Because of the lack of data on patents, it is not possible to address this question directly. The thesis therefore focuses on a critical aspect of patents: their inventiveness. ¶ The main contribution of this thesis is a detailed empirical assessment of the inventiveness of patents. This assessment breaks new ground by using the actual claims in the patent specification as the basis for a qualitative assessment against the yardstick of whether there is any new contribution to knowledge. This yardstick is used because a key social benefit from private invention is the spillovers from new knowledge. In addition a low inventive threshold encourages monopoly grants for inventions that would have occurred absent patents, and thus increases social costs without any offsetting benefits. ¶ ...