The most fundamental requirement of patent law is that a patented invention must be new. Given the longstanding, foundational nature of this novelty requirement, one might expect its contours to be well settled. And yet some of its most basic aspects remain unresolved. At the center of these unresolved issues lie what we term “real-world prior art.” In patent law, prior art is something that predates an invention and may render it not new. “Real-world” prior art activities involve using or selling real-world embodiments of the invention. Consider a few examples. Suppose Aleida demonstrates her invention to members of the public but does not allow them to touch it. Has she put the invention into “public use,” thus preventing others from obta...