History isn't just the residue of forces too vast to be altered by individual endeavor. It also is the product of chance events and calculated decisions.

President George H.W. Bush's selection of David Souter for the Supreme Court of the United States, a product of both chance and miscalculation, changed the course of American law over the past two decades - in ways largely at odds with the hopes of those most responsible for his selection. Justice Souter's service on the Court over the past 19 years - helping decide cases like Planned Parenthood v. Casey (the 1992 decision reconfirming and reconstructing federal abortion rights) and Kelo v. City of New London (a 2005 opinion expanding government rights over private property) - has left liberal groups who originally opposed him far happier than the conservatives who had supported him.

President Barack Obama's choice of a replacement could have similar repercussions for the law over the coming years and for some of our political institutions as well. But the coming fight over selection and confirmation should be waged with a few cautions in mind - less for how this fight ends (with Senator Arlen Specter's admission that he's more ass than elephant, the outcome is predictable enough) than for its reverberations down the road.

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First, don't think that the selection is irrelevant because President Obama is certain to replace a liberal justice with another liberal, leaving the "balance" on the Court intact. Even if justices only came in two flavors, with no other differences than the L or C label, having a justice ten or fifteen or twenty years younger makes a huge difference in the expected composition of the Court ten or fifteen or twenty years from now.

Second, the whole notion of "balance" is fundamentally off target. Justices don't fit simple, bipolar patterns. Justices Scalia and Thomas, for instance, disagree vigorously over questions of deference to administrative agencies. Justices Breyer and Ginsburg part company over critical aspects of intellectual property rights. In its 2002 decision in Verizon v. FCC, Justice Breyer wrote the lone dissent - joined only by Justice Scalia. Justice Souter, despite the liberal label, wrote opinions such as MGM Studios v. Grokster (2005) and Bell Atlantic v. Twombly (2007) that have been applauded by most of the conservative business community.

Justices differ along many dimensions. They take different approaches to interpretation, lean in different directions on issues of administrative law, criminal law, federalism, national security power, and individual rights. Even within a broadly defined category, justices will differ. At bottom, law is complicated and the threads that run through it form patterns far more complex than any set of labels can hope to capture - which is equally true for the judges themselves.

Beyond the cross-cutting substantive and methodological divisions, justices have different personalities and different strengths. Some are gregarious, some diffident, some sharper writers, others better grounded in practical judgment. These at times have greater impact than commonly attributed ideological affinities. Justice Souter is an incrementalist, uncomfortable with grand theory, and profoundly disconnected from many aspects of social life that help shape other justices' views. In the end, neither the work of the Court nor assessments of prospective justices' performance on it are as simple as pundits and politicians make them seem.

Third, think beyond today. The politics of judicial appointments always is framed in immediate terms. But the issues that define a justice's legacy often are entirely invisible at the time he or she is appointed. A majority of the Court that decided Roe v. Wade (1973), for example, consisted of justices appointed by Presidents Johnson, Kennedy, Eisenhower, and Franklin Roosevelt - none selected with an eye to deciding cases on abortion rights. What can be foreseen is the general difference that particular approaches to deciding cases will bring and the difference that having justices of intellectual depth, judicial temperament, and congeniality will make for how the Court does its work. Those concerns, not the predicted outcome of any particular vote, should guide both the President and the Senate.

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Players in the drama to come also should be respectful of the institutions their decisions will affect. Republicans should avoid reflexively partisan attacks on the President's choice - after eight years of asserting that naming judges is, first and foremost, the President's choice.

Democrats, for their part, should remember their outrage over not being consulted about potential nominees when they were the minority party and their insistence that the Senate exercises a role in appointments "co-equal" to the President's. Consider, for instance, the words of Senator Barack Obama during debates over Justice Sam Alito's nomination just a few short years ago:

"There are some who believe that the President, having won the election, should have the complete authority to appoint his nominee, and the Senate should only examine whether or not the Justice is intellectually capable and an all-around nice guy. That once you get beyond intellect and personal character, there should be no further question whether the judge should be confirmed.

"I disagree with this view. I believe firmly that the Constitution calls for the Senate to advise and consent. I believe that it calls for meaningful advice and consent that includes an examination of a judge's philosophy, ideology, and record."

Senator Obama was a partisan on judicial appointments, voting against highly qualified nominees like John Roberts and Sam Alito because he disagreed with some of their decisions.

President Obama, on the other hand - a self-declared "post-partisan" President - should want a confirmation process that intrudes less into judicial decision-making. While a 60-vote Democrat majority helps, so would a nomination that invites a change of tone for this process. If he nominates someone whose approach to interpretation is flexible enough to accommodate any politically or personally preferred result, the President risks increasingly politicizing the courts and undermining the rule of law. Rabid partisans may applaud, but moderates who helped elect him will not. Even the most Teflon politician can have sticky patches, and the wrong selection here could peel away at least a bit of protective covering.

Such a miscalculation, however, might have one final irony. If the President makes too aggressively liberal a pick, then Justice Souter's resignation could be the catalyst to unite and reenergize a Republican Party he had forsaken - which would bring the Souter story full circle.