"Our best security, our only security, is in the world of ideas, and I sense a slight foreboding," he said.

Kennedy, a moderate justice who has become a key swing vote on the Supreme Court, argued that the meaning of the phrase "rule of law" must be made clear in order to spread the cause of freedom to other countries. He avoided singling out specific nations.

He said the rule of law has three parts: it must be binding on all government officials, it must respect the dignity, equality and human rights of every person, and it must guarantee people the right to enforce the law without fear of retaliation.

"Americans must understand that if the rules of law have meaning, such as hope and inspiration for the rest of the world, it must be coupled with the opportunity to improve human existence," Kennedy said.