On what basis can we argue for reforming politics, society, or the economy? What must our arguments address? How should our arguments be structured? Under what circumstances are these arguments, if adopted, most likely move society in the right direction? In Tyranny of the Ideal: Justice in a Diverse Society, philosopher Gerald Gauss lays out a rigorous and widely useful exposition of what a philosophy focused on justice needs to do in order to shift from the realm of pure moral philosophy—“understood simply as the justification of normative judgements” (p. 17)—to making recommendations about the political world. This book is not concerned with advancing any particular philosophy of justice; rather, the book explores the nature and structure of arguments that draw on these philosophies, as they must, to make recommendations about political and social reform. If this seems like an important and intriguing question, then you will enjoy reading this book. If this seems like a straightforward or a pointless question, then this book is just the medicine you need.

Gaus begins by rejecting the notions that justice can be reduced to “deep intuitions” or “pure proceduralism.” He argues instead that we must consider how a conception of justice would come into being in the political world. If some conception of justice was perfectly implemented, what would the world look like? In considering this question, institutional structures and the characteristics of populations matter. And tradeoffs between desiderata are inevitable. This “social realization condition” faces the messy complexity of social phenomena. Gaus outlines the parameterization of simplifying models in terms that will be familiar to social scientists. A model of the ideal, Gaus writes, should be “ … our best estimate of what a real, ideal world would actually look like, not an impossible world that we use to judge our own” (p. 38). This model orients the quest for justice in the domain of attainable worlds, which includes the status quo, the ideal, and an unimaginably large number of intermediate cases.

Once a set of evaluative criteria are established, the argument requires a mapping function for applying these criteria to the justice relevant features of attainable worlds. The mapping function weights each criterion by their centrality to the theory, and takes account of uncertainty in the models. This creates an “expected justice score” that enables comparison between alternatives. This mapping function is necessary, but two things make it insufficient for judging alternative worlds. First, as Gaus stresses, the distance between social worlds—which corresponds roughly to the degree of similarity between those worlds—fixes the difficulty of moving from one world to another. It is easier to move to closer, more similar worlds than to farther, more different ones. Second, the justice relevant features of social worlds are interdependent. More of some features implies less of others. Thus, the pursuit of justice cannot proceed feature-by-feature, as it could if no interdependencies existed. Taken together, the pursuit of justice is not synonymous with a straightforward climb to increasingly just outcomes. The domain of possible social worlds is a “moderately rugged” high-dimensional landscape punctuated by hills (local maxima) and valleys (local minima). We see the worlds nearest to us more clearly than those far off. But maximizing justice in the neighborhood may be a move away from the better world, concealed just off in the distance, or perhaps way off in another direction, by the fog of uncertainty. We are thus faced with a choice: Do we go climb the hills we can see, or do we trust our maps to the ideal?

In the final chapters, Gaus considers this question from the vantage point of the kind of society most conducive to surveying the landscape of alternative social states. Gaus begins by developing the wisdom-of-the-crowd argument, with the Hong-Page corollary of how a diversity of perspectives creates a “handing-off-the-baton dynamic” that improves the search for justice. A diversity of perspectives surveys more of the landscape and helps each perspective to overcome its local optimum. Thus, in this “Diversity Trumps Ability Theorem,” a large group of diverse perspectives will do better than a homogenous group of the “best problem solvers.” Gaus finds this approach helpful, but ultimately less helpful than it first appears for the pursuit of ideal justice. Each perspective on justice explores this high-dimensional landscape by heading out in a different direction from the status quo. Thus, diverse perspectives expand and clarify our view of the neighborhood in which we are situated. But diversity does not lead us, as if by solving an optimization problem, to the promised land of ideal justice. Diverse perspectives generate uncertainty about the direction of the ideal, and thereby undermine the impetus for far flung expeditions into the foggy horizon. For these reasons, Gaus argues that it is the Open Society, mired in disagreements about justice, and not the tyrannizing “mirage” of a well-ordered utopia, that helps us to move in the right direction.

The Tyranny of the Ideal takes political philosophy to the doorstep of evidence-based social science. Bring your notebook. This is not bedtime reading. Here, as in his earlier work, Gaus clarifies our thinking in the best tradition of philosophy. The book is rigorous, useful, and, without question, timely.