P developed a computer program and telecommunications network designed to facilitate securities trading. In March 1995, he leased the system to R. M. Stark & Co., a securities brokerage. A little over a year later, he applied for a patent for an interactive securities trading system that was based substantially on the leased product. The U.S. Patent and Trademark Office issued the patent in January 2000. P filed a patent infringement suit and was represented by D. The patent was declared invalid under the “on sale” bar, 35 U.S.C. §102(b). The District Court granted the summary judgment motion and declared P's patent invalid. P then filed a motion for reconsideration arguing for the first time that the lease agreement was part of ongoing testing of TEXCEN and therefore fell within the “experimental use” exception to the on-sale bar. The district court denied the motion. P, convinced that his attorneys' failure to raise the experimental-use argument earlier had cost him the lawsuit and led to invalidation of his patent, brought this malpractice action in Texas state court. D defended on the ground that the lease to Stark was not, in fact, for an experimental use, and that therefore P's patent infringement claims would have failed even if the experimental-use argument had been timely raised. The trial court agreed and granted summary judgment to D and the other lawyer defendants. On appeal, P claimed because his legal malpractice claim was based on an alleged error in a patent case, it “arises under” federal patent law for purposes of 28 U.S.C. §1338(a). And because, under §1338(a), “no State court shall have jurisdiction over any claim for relief arising under any Act of Congress relating to patents,” the Texas court--where P had originally brought his malpractice claim--lacked subject matter jurisdiction to decide the case. A divided panel of the Court of Appeals of Texas rejected P's argument. It held that the federal interests implicated by P's state law claim were not sufficiently substantial to trigger §1338 “arising under” jurisdiction. The Supreme Court of Texas reversed. The Supreme Court granted certiorari.