Judge Richard A. Posner, of the U.S. Court of Appeals in Chicago, is one of America’s most prolific intellectuals and legal philosophers. Following the September 11, 2001, attacks, his powerful mind turned to the challenge of equipping government for the new threat environment, where sneak attacks by transnational terror networks have supplanted conventional national armies as the chief danger.

This study produced a series of books — including “Catastrophe” (2004), “Preventing Surprise Attacks” (2005), and “Uncertain Shield: The U.S. Intelligence System in the Throes of Reform” (2006) — which assessed the challenge from not only a legal perspective but the disciplines of intelligence and economics. The law, however, is the principal focus of Mr. Posner’s latest contribution. “Not a Suicide Pact: The Constitution in a Time of National Emergency” (Oxford University Press, 171 pages, $18.95), is a short, accessible, and provocative analysis of the tension between security and liberty during crisis.

The apt title is drawn from a 1949 dissent by Supreme Court Justice Robert Jackson, who warned that a failure to “temper … doctrinaire logic with a little practical wisdom” would risk turning the “Bill of Rights into a suicide pact.” This is the heart of Judge Posner’s legal philosophy, pragmatism. Best explicated in his 1995 book, “Overcoming Law,” Judge Posner’s approach is, as he puts it,”instrumental”: practical, not dogmatic. His chief concern is what works empirically. Formalistic methods of deduction from the Constitution’s text and adjudicative theories sprung from “metaprinciples” (such as Jon Hart Ely’s “representation-enforcing” judicial review, Justice Antonin Scalia’s “originalism,” or Justice Stephen Breyer’s “active liberty”) are regarded with skepticism.

For Judge Posner, the Constitution, though nearly 220 years old, continues to provide pragmatic solutions for emerging problems because its injunctions (not only in the Bill of Rights but in the articles that create governmental powers) are nebulous — framed in the language of “due process,” “liberty,” “unreasonable searches,” and the like. This provides a wide interpretive berth. Thus, because the Constitution is difficult to amend, and because they are immune from being reversed, Supreme Court justices are the engine of a continuous “aggiornamento.” Over time, the Court is better understood as having created “rights” than having discovered them latent in the Constitution’s text.

Important corollaries flow from this arrangement. First, it requires a healthy judicial humility, recognizing both the limits of judicial competence and the dynamic march of time, which has a way of undermining even the most self-assured premises on which today’s decisions are made. Rights do not exist in a vacuum but compete with the values represented by other rights. It is when jurists, heedless of tomorrow’s challenges, become doctrinaire that they are most apt to err — when, as Judge Posner puts it, they place “a thumb on the constitutional balance” by inflating situational preferences (such as judicial warrants in search cases, or concepts of “imminent harm” and “prior restraint” in cases involving regulation of speech) into immutable rules. Second, there is a significant difference between the core values rights protect and the scope of that protection. The latter is quite elastic. Combined with the judicial propensity to circumvent straitjacketing precedents by drawing factbased distinctions, this elasticity makes the Constitution adaptable to new crises.

Judge Posner’s pragmatic approach to the current crisis is to recognize that, while international terrorism is sui generis in that it does not fit precisely into either the “war” or “law enforcement” paradigm, it is closer to war. It requires for national security robust governmental powers to gather intelligence, monitor the religious exercise of jihadists, detain operatives, and conduct trials (whether they are technically “military tribunals” or not) that deny terrorists the full benefits of the criminal justice system. Judge Posner would worry far less about whether the NSA’s interception of al Qaeda’s international phone and email communications violates the 1978 Foreign Intelligence Surveillance Act (FISA), and far more about how FISA’s obsolescence can be remedied so it is up to the modern challenge of identifying potential terrorists rather than simply surveilling the ones already known to be dangerous. If the goal is prevention of attacks, waiting to develop “probable cause” means waiting too long.

Most intriguing is Judge Posner’s grappling with “the law of necessity.” The specter of mass-destruction weapons that could dwarf the carnage of 9/11 means government, in an emergency, may need to coerce information by torture. Plainly, the Constitution has created an executive branch with the power to do this, but should it have the authority? Should we acknowledge necessity by providing for it in our law? Judge Posner thinks not. Our blanket prohibition, he believes, will not (and should not) prevent drastic measures when the time truly calls for them, but remains the best insurance against their being employed unnecessarily. Judge Posner suggests that this may be an example of a “law of necessity” that trumps the Constitution; I think it is better understood as an aspect of constitutional power that defies regulation.

Also noteworthy is Judge Posner’s withering critique of civil liberties extremists, whose distrust of government is “excessive” and whose disparagement of the terrorist threat is “irresponsible.” In their ardor, the author notes that civil libertarians fail to see their deep self-interest in capable security. Security guarantees their rights. A terrorist attack perceived as owing to an undue handcuffing of government would inevitably result in a roll-back of our liberties.

As always, Judge Posner has given us much to ponder on the day’s most urgent questions. Critics right and left will disagree about many of the details. Beyond that, for all Posner’s admonitions about judicial restraint, he is content to rely on judges to restrain themselves from second-guessing the political branches’ conduct of warfare. About that, we could have more confidence if all judges were Judge Posner. But, as this excellent book once again shows, there is only one.

Mr. McCarthy, a former federal prosecutor, is a senior fellow at the Foundation for the Defense of Democracies.