At one point, Senator Charles Grassley, a Republican of Iowa, noted with scorn that Harvard Law School, where Ms. Kagan had been dean, required first-year students to study international law. Senator Tom Coburn, a Republican of Oklahoma, asked why Ms. Kagan thought it was acceptable to use foreign law to interpret the Constitution, which she retorted was almost never the case. Senator Jon Kyl, a Republican of Arizona, summed it up: “I’m troubled by it,” not because foreign law would create a United States precedent, but “because it suggests that you could turn to foreign law to get good ideas.”

In her remarks on Friday, delivered to the International Academy of Comparative Law at American University in Washington, Justice Ginsburg said that kind of thinking is completely at odds with the views of the nation’s founders, who were extremely interested in the opinions and laws of other countries. The authors of the Federalist Papers, she noted, cited the “high importance” of observing the “laws of nations.” And, of course, the Declaration of Independence itself was an appeal to the “opinions of mankind” in a “candid world.”

To the extent that the United States wants its ideals and legal system to inspire others, it should take interest in ideas from overseas, she said, not necessarily adopting them but drawing on them. Ms. Kagan made it clear that foreign opinions are not authoritative, Justice Ginsburg said, adding: “They set no binding precedent for the U.S. judge. But they can add to the store of knowledge relevant to the solution of trying questions.”

In 2002, the Supreme Court prohibited the execution of those mentally retarded, noting that the practice is overwhelmingly disapproved around the world. In 2003, it struck down prohibitions on gay sex, which it called “an integral part of human freedom in many other countries.” In 2005, the court prohibited the execution of minors, again noting global opinion. “It does not lessen our fidelity to the Constitution or our pride in its origins,” Justice Anthony Kennedy wrote in that case, to acknowledge “the express affirmation of certain fundamental rights by other nations and peoples.”

The reasoning in those cases was greeted with catcalls from legal isolationists, as no doubt will be Justice Ginsburg’s brave speech. Foreign law will undoubtedly be cited this week as a reason why many Republicans will vote against Ms. Kagan’s confirmation. They might want to re-read James Madison’s description in the Federalist Papers of the ideal legislator: “He ought not to be altogether ignorant of the law of nations.”