The United States Supreme Court has the last word on the meaning of the Constitution in our political system. In a real sense, the Constitution is what the Court says it is. But what the Court says has changed greatly over the years. In 1896, for instance, the Court announced in Plessy v. Ferguson that the “separate but equal” racial segregation of railroad cars was constitutionally acceptable, but in 1953, in Brown v. Board of Education, it rejected that doctrine. In 1986, in Bowers v. Hardwick, it held that the Constitution allowed states to criminalize same-sex sexual activity, but in 2003, Lawrence v. Texas reversed Bowers. Even at any given moment there are typically sharp divisions among the Justices on important questions of constitutional interpretation: of the decisions just listed, only Brown was unanimous.

How can these disagreements be explained? The simplest theory starts with the premise that the Constitution prescribes clearly correct answers in all cases. Some Justices faithfully adhere to those solutions and others seek instead to advance their own political agendas.

This was basically the position of Justice Antonin Scalia, and his aggressive advocacy in its favor has exerted an enormous influence on the public’s perception of the Court and the Constitution. Unfortunately, that influence is largely negative because the position is wrong. It takes no more than a quick glance at the words of the Constitution to see that it does not, in fact, contain specific answers to every question that might arise. Instead, the Constitution frequently speaks in majestic generalities. Equal protection, due process, the freedom of speech—these are general concepts. The drafters of those words explained them in terms of broad notions of fundamental fairness and justified distinctions rather than arbitrary or oppressive discrimination. The ratifiers argued bitterly about which government acts would be forbidden and which allowed. It is not really possible to deny that on contentious constitutional questions there is some open space for argument, some indeterminacy, within which opposing positions are both legally plausible. So what determines the choices that Justices make?

One answer, also consistent with the Scalia model, is that when there is no answer, everyone acts like a politician. They advance the interests of their favored political party, or whatever policies they like, even when there is no constitutional basis for the choice.

Judges of course deny this. They claim to be umpires rather than players. As James D. Zirin puts in it in this timely book, they “fall all over themselves saying” that when they decide constitutional cases, they are merely applying the law. But Mr. Zirin argues, to use the words of Justice Scalia, that this position is “pure applesauce.” When it comes to certain highly charged issues such as homosexuality, abortion, affirmative action, illegal immigration, capital punishment, and guns, he contends, the opinions of the Supreme Court reflect the views of the Justices arising out of personal life experiences, gender, religion, or political predisposition: in short, identity politics.

t is true, almost as a matter of logic, that if law or the Constitution do not take judges all the way to an answer, something else must be at work. And it is also true, I believe, that life experiences and predispositions are in large part the something else that enters judicial decision- making. But how exactly does that happen, and what are we to make of it? I believe the picture is a complicated one.

First, some Supreme Court decisions—perhaps most notably Bush v. Gore—are hard to explain in any terms other than partisanship. Those decisions are justifiably condemned. But not all decisions, not even all of the controversial and divisive ones, are driven by partisan politics.

Instead, what determines a Justice’s position in many cases is better described as either a constitutional (not a political) ideology, or a general worldview. There have been divisive cases in recent years, for in- stance, about the scope of federal legislative power. The Justices clearly have commitments and preconceptions about this issue, but those commitments and preconceptions are part of their constitutional interpretation. Some Justices think that the Founding and the original Constitution struck a particular balance between state and federal authority and that the Court should try to preserve that. Others think that the Reconstruction Amendments, the 13th, 14th, and 15th, forever altered that balance, so that the Founding conception is no longer part of our Constitution. That is ideology, maybe, but it is still rooted in an understanding of the Constitution.

Justices also differ on the question of how much they trust judges–how willing they are to commit difficult choices to the judiciary rather than the political process. Justice Scalia had a very dim view of judicial capacity to settle hard moral questions; Justice Kennedy has much more confidence in it. Justices differ similarly on how much they trust the executive branch to take action in the name of national security without judicial oversight. Here, Justice Thomas is probably the most deferential. The liberals are less so, and Justice Scalia charted a compromise position, varying his trust depending on whether the action was directed at American citizens or not. Again, these are not narrow partisan differences. They reflect in part the Justices’ weighing of underlying values—liberty vs. security, for example—and in part their faith in the men and women who make up the different branches of our government. The Justices are not always consistent in these views from case to case—and when they are inconsistent, we might suspect that partisan-ship is present—but to the extent that they are consistent, partisanship is not the driving force.

Last, the Justices differ in their understanding of general facts about the world. How much of a threat is in-person voter fraud? How hard is it for people to get the kinds of identification required by voter-ID laws? Are abortion protesters interested in thoughtful communication or intemperate harassment? Can Southern states be trusted not to attempt to disenfranchise minority voters? Are blacks, women, and the LGBT population fundamentally the same as straight white males, or are they different in ways that might justify differential treatment? These are the questions on which important recent constitutional cases have turned. Answers to them tend to correlate with political ideology—which is why the votes of the Justices do too—but they do not come directly from it. More likely, these underlying views about the world produce both political ideology and votes in constitutional cases.

Where do underlying views come from? They’re informed by research and the arguments of parties in particular cases, of course, but they’re shaped first and foremost by life experience. That means that a Justice’s background, race, and sex will affect his or her constitutional judgments. But does that make them partisan, or the product of identity politics in a negative sense?

In this book, James Zirin makes the argument that the answer is yes. He examines the personal factors that influence judges through a wide-ranging and detailed study of Justices and cases. He suggests that such influences are generally illegitimate, that they make the Supreme Court “a political court.” I tend to think that instances of true partisanship are the exception rather than the rule, that most decisions can be explained in nonpartisan terms as the product of influences judges may properly take into account. But regardless of which view readers accept, they will benefit greatly from reading this book. It will deepen their understanding of the Court at a time when concerns of partisanship and identity politics undeniably overwhelmingly drive the nomination process. All Americans should pay close heed.

Kermit Roosevelt is a professor of constitutional law at the University of Pennsylvania Law School and the author of The Myth of Judicial Activism and the novels In the Shadow of the Lawand Allegiance.