Along with Robert Bork, Antonin Scalia was the leading proponent of originalism and textualism. Richard Posner is his nemesis. His position represents the polar opposite of Bork and Scalia's.
As one NRO pundit recently put it, "In constitutional theory, individual rights come from our Creator. They preexist the Bill of Rights. Constitutional guarantees merely safeguard our rights; they do not grant our rights in the first place."
Compare that to Posner:
The argument of The Problematics of Moral and Legal Theory is that when we call practices “immoral” we do so in reference to our own values. The people who make this argument, people like Richard Rorty and me, are not immoralists; we are pragmatists; we simply believe that there is no reliable external perspective from which to evaluate competing moralities. Societies that practice infanticide do not regard infanticide as immoral, and we civilized Americans cannot say they’re “wrong” to do so unless we add—by our lights.
Posner's judicial philosophy is eclectic, which makes it hard to pigeonhole. He views humans are monkeys with big brains.
It's been variously classified as legal pragmatism and the economic analysis of law. On social issues he can be by turns libertarian or authoritarian. One thing that comes through on national security issues is his commitment to consequentialism.
What's interesting about Posner is that he embodies a central dilemma of liberal jurisprudence. Back in 2014 he authored a ruling striking down gay marriage bans. That was widely lauded by liberal pundits.
However, back in 2006, he authored a book, Not a Suicide Pact, that rubber-stamped nearly all of the Bush-era counterterrorist measures. Civil libertarians were outraged.
He espouses a "Living Constitution". He thinks the Bill of Rights oscillates between provisions that are either vague and uninformative, or specific, but antiquated and obsolete. As a result, civil rights or Constitutional rights are ultimately a judicial construct.
Since he doesn't consider the text, logic, or intent of the Constitution to be normative, what is the guiding philosophy that informs Constitutional rights? Well, when it comes to national security, consequentialism is his guiding philosophy. Balancing liberty and security.
This stood in stark contrast to civil libertarian critics of the Bush administration, who take a more absolutist view of the Bill of Rights. Yet many of his critics share his commitment to a Living Constitution. Indeed, that's why they can conjure a right to homosexual marriage in the Constitution. Posner's jurisprudence exposes the contradiction in their own position. The civil liberties codified by the Bill of Rights are either absolute or relative, "inalienable" or conventional. If absolute, you can't combine that with a "Living Constitution". If relative, you can't take issue with Posner's utilitarian approach to the rights of terrorists.