No one doubts that the Supreme Court is ideological, at least in part. No other heuristic can so effectively predict and explain the behavior of justices. And political figures engaged with the Court do not merely operate in space identified by liberal and conservative ideologies; they seem to inject ideology straight into their veins.

While we may agree that ideology matters, we often struggle to explain what exactly it is. Are liberals supposed to favor free speech? Even for corporations? Or pro-lifers? Are conservatives opposed to federal government regulations? Even when they preempt state environmental regulations?

Lawrence Baum’s Ideology on the Supreme Court provides a thoughtful, and thought-provoking, argument about the true nature of ideology on the Court. Baum argues that we need to pay more attention to the group basis of judicial decision making. That is, ideology matters, but the way it matters depends profoundly on how groups are affected by court decisions.

To get a sense of Baum’s argument about how ideology matters, it is useful to begin by noting what ideology is not. First, ideology is not logic. Conservatives did not syllogize their way to their world view; liberal policy preferences did not emerge deductively from a short list of axioms. Instead, ideology reflects the shared views of major political actors. Conservatism is what conservatives believe; liberalism is what liberals believe.

Where do these beliefs come from? This leads to a second fundamental aspect of ideology: Ideologies evolve. There are two primary facets of this evolution. The first, as Hans Noel (2013) states in Political Ideologies and Political Parties in America, concerns “what goes with what.” At one point, being pro-business went along with favoring a more active federal government, more protection for African Americans, and higher tariffs. Now, being pro-business is seldom associated with activism on behalf of African Americans. Ideologies package views together for political actors.

The second way ideologies evolve is with regard to the specific content of ideological positions. In the 1940s, racial conservatives opposed integration of the armed forces and federal efforts to reduce lynching. Today, racial conservatives oppose affirmative action and want to reduce immigration.

Baum provides a fascinating look at the evolution of ideology on the Supreme Court by exploring historical linkages between issue positions and ideology across issues. He focuses on free speech, crime and takings, and also examines other issues. He debunks the view that ideological behavior by justices on these cases is a high-minded, conceptual exercise. On each issue, liberals and conservatives varied across time in how they approached cases. On speech, for example, liberals in some periods were the most willing to strike down laws and other government actions on the basis of free speech; in other periods—such as the contemporary era— conservatives have more actively used the first amendment to strike down government actions.

What explains these variations? Baum argues that it is largely group affect. On speech, liberals love speech—as long as it is by unions or communists or other groups liberals like. Conservatives love speech by businesses. And the evolution of case law has been largely driven by the kinds of groups claiming that their speech rights have been violated.

The regulation of antiabortion protests also provides an excellent example of Baum’s point. Liberals cut their teeth on protest cases defending the rights of unions, war opponents, and others to use public space to make their claims, even at the risk of public disorder. When conservative antiabortion protestors came to the Court for protection, liberals ceded free speech to conservatives. What explains the change? It is not hard: The groups affected changed, and so too did the ideological politics of the Court.

Baum’s chapter on crime will surprise many readers. We typically view crime as a prototypical left–right issue. In fact, liberals were tougher on crime for some periods than were conservatives. This seems puzzling at first, but the author’s approach resolves the puzzle decisively: Liberals were tough on crime in the early twentieth century when the criminal cases making their way to the Court were largely about convictions of businesses and white-collar criminals. On violent crimes, the left–right dimension has always been with us, and on those cases liberals, who are more sympathetic to the lower-status and often nonwhite accused, have always been more willing to overturn convictions.

Baum’s project extends the insight from David Karol’s (2009) Party Position Change in American Politics, that politicians are more consistent about who is good than what is the good policy to the Court. However, we should not believe (and Baum does not believe) that this reduces the Court to a simple pluralist legislature advancing group interests.

First, the author focuses on the connection between ideology and issues, such as free speech and crime. He focuses less on how underlying values influence which groups justices valorize. This decision is deeply ideological. Most justices on the Court have been wealthy Christian men. Many, however, fought for poor people, labor unions, nonmainstream religions, and women. Their affinity for these groups was not interest or origin based, but rooted in ideological and other values. Or, to take another example, Clarence Thomas is an African American who rose to the Court from modest circumstances. From Baum’s perspective, Thomas’s positions can frequently be explained on a group basis as favoring outcomes that benefit big business and hurt Civil Rights groups. And yet we should not forget the important values that led Thomas to favor an ideology that favors the groups he favors.

Second, while Baum convincingly shows the gravitational pull that group interests exert on justices, we should keep in mind the mechanisms that allow justices to rise above narrow group concerns. Justices do not simply state: “Big business is awesome; let’s do what they want,” or “Anything good for the Jehovah’s Witnesses is fine by me.” Instead, justices justify their decisions with opinions that are organized around legal or moral principles.

So even as justices may amend, distinguish, or clarify their principles to advance their favored groups under new circumstances, they may also simply stick to their guns. Their daily lives are only indirectly affected by whether a student can have a sign that says “Bong hits for Jesus”; their daily lives are directly affected by the writing and argumentation of lawyers, clerks, and their peers about legal issues. There is no shortage of cases in which justices have come down on the sides of groups that they all find objectionable (such as hecklers at funerals) or that they individually find unpleasant (such as Antonin Scalia’s defense of flag burners or accused criminals subject to judge-applied extensions of prison sentences).

Ideology is a complex phenomenon with many moving parts. The characteristics of cases are dynamic: Cases move left and right on individual dimensions and can combine with a dizzying array of dimensions. So, too, are the values of individual justices, both in where they stand and how they combine issues. Ideology on the Supreme Court advances and supports a novel claim, even as Baum recognizes these subtleties. His book is smart and will influence this conversation for a long time.