Richard A. Posner is a judge on the U.S. Court of Appeals for the 7th Circuit and a senior lecturer at the University of Chicago Law School.

The decision of the Republican Senate majority to consider no nominee of President Obama to fill the vacancy on the Supreme Court created by the death of Justice Antonin Scalia is significant, but not for the usual reasons given — that the work of the court will be disrupted or that the senators are showing disrespect for the president by refusing to consider any nominee he might name. All that happens when the court is reduced to an even number of justices (eight in this instance) is that a few key cases are scheduled for reargument in the court’s next term, which will begin in October. A few months later, after the new president has taken office, the vacancy will have been filled.

Rather, the significance of the Senate’s action lies in reminding us that the Supreme Court is not an ordinary court but a political court, or more precisely a politicized court, which is to say a court strongly influenced in making its decisions by the political beliefs of the judges.

This is not a usurpation of power but an inevitability. Most of what the Supreme Court does — or says it does — is “interpret” the Constitution and federal statutes, but I put the word in scare quotes because interpretation implies understanding a writer’s or speaker’s meaning, and most of the issues that the court takes up cannot be resolved by interpretation because the drafters and ratifiers of the constitutional or statutory provision in question had not foreseen the issue that has arisen. This is notoriously the case with respect to the Constitution, composed in 1787, and the Bill of Rights, composed two years later. But it is also the case with respect to the 14th Amendment, composed in 1866 and ratified two years later; and in the statutory realm, it is the case with respect to numerous old but still influential statutes, such as the Sherman Antitrust Act of 1890, and countless modern statutes as well. Eighteenth- and 19th-century politicians, and many 20th-century ones as well, did not foresee or make provision for regulating electronic surveillance, sound trucks, flash-bang grenades, gerrymandering, child pornography, flag-burning or corporate donations to political candidates.

When judges are not interpreting, they’re creating, and to understand judicial creation one must understand first of all the concept of “priors.” Priors are what we bring to a new question before we’ve had a chance to do research on it. They are attitudes, presuppositions derived from upbringing, from training, from personal and career experience, from religion and national origin and character and ideology and politics. They are unavoidable tools of decision-making in nontechnical fields, such as law, which is both nontechnical and analytically weak, in the sense that there are no settled principles for resolving the most difficult and consequential legal controversies. The tools I am calling priors can in principle and sometimes in practice be overridden by evidence. But often they are impervious to evidence, being deeply embedded in what we are, and that is plainly true of judging — not in every case but in cases that can’t be resolved by interpretation or some other decision-making tool that everyone understands and uses in an identical way.

Sen. Patrick J. Leahy (D-Vt.), the Senate Judiciary Committee’s top Democrat, used his opening statement at the Justice Department oversight hearing to criticize how Republicans decided to take no action on an Obama nominee to replace the late Justice Scalia. (Senate Judiciary Committee)

The priors that seem to exert the strongest influence on present-day Supreme Court justices are political ideology and attitudes toward religion. It is well-understood that there are now, with Scalia’s death, three very conservative Catholic justices (Samuel A. Alito Jr., John G. Roberts Jr. and Clarence Thomas), four liberal justices (Stephen G. Breyer, Ruth Bader Ginsburg, Elena Kagan and Sonia Sotomayor) and a swing justice (Anthony M. Kennedy) who is generally conservative but liberal in several important areas (such as gay rights and capital punishment of minors). Before his death, Scalia, a solidly conservative and devoutly Catholic justice, gave the conservatives a definite majority on the court that pushed the court in a conservative direction, much as the liberal justices of the 1950s and 1960s had pushed the court in a liberal direction. (Kennedy and Sotomayor are also Catholic but less influenced by religion; the three liberal justices besides Sotomayor are Jewish but not, it seems, influenced by Judaism in their judicial work.)

President Obama might nominate to the Scalia vacancy a centrist, or even a conservative-seeming judge of sterling qualifications, who yet might be a “stealth” liberal in the mode of John Paul Stevens, David Souter, Harry A. Blackmun and to a lesser extent Sandra Day O’Connor — justices who were or at least seemed conservative when appointed but became significantly less so as justices. Republican senators can avoid the embarrassment of confirming a stealth liberal by refusing to hold a confirmation hearing for any Obama nominee, hoping that the next president will be a Republican and will appoint someone in Scalia’s mold. The Republican senators’ behavior is proof (were any needed) of the Supreme Court’s politicization.

I may seem to be criticizing the court by calling it politicized. That is not my intention. When a statute or constitutional provision is clear, judges (including justices of the Supreme Court) will usually apply it to disputes within its scope, whether they like it or not. But when there is no clarity in the relevant provision — when the judges are on their own — their priors will tug them this way or that, and the tug may be decisive. That is inevitable, and bowing to the inevitable is not misconduct, however much it deviates from the “official” — the self-protective “the law made me do it” — conception of judicial decision-making.