His domestic libertarianism provides no philosophical foundation for most of the federal government. As a practical matter, he can call for the end of Obamacare but not for the abolition of Medicare or Medicaid or the National Institutes of Health. Yet these concessions to reality are fundamentally arbitrary. The only principle guiding Paul’s selectivity is the avoidance of gaffes. Of which he is not always the best judge.

The same is true of Paul’s “constitutional foreign policy,” which he now calls (as evidence of his evolution) “conservative realism.” There is no previously existing form of “realism” that urges a dramatically weakened executive in the conduct of foreign and defense policy — which is Paul’s strong preference. He denies the legal basis for the war on terrorism, warns against an oppressive national security state and proposes to scale back American commitments in Europe, Asia and the Middle East. Paul is properly described as a libertarian noninterventionist.

His father, Ron Paul, is gleefully specific in his charge that American aggression creates the “blowback” of terrorism. The son qualifies the argument without repudiating it. “Some anger is blowback,” he now says. In 2009, he called his father’s theory a “message that can be presented and be something that Republicans can agree to.” A recommended reading list posted (briefly) last year on Paul’s Senate Web site included Chalmers Johnson’s “Blowback: The Costs and Consequences of American Empire” and Ron Paul’s “A Foreign Policy of Freedom.”