As Christopher Bobonich (1993, p. S92) reminds us, “The idea of basing an ethical theory on human nature has attracted Western philosophers from the very beginning of philosophical reflection on ethics.” Unfortunately many philosophers have not kept apace with the best research in the empirical disciplines exploring that topic, with the result that their works contain more impractical idealism than they intend. In order to ensure the soundness and persuasiveness of their ethical conclusions, philosophers should routinely confront the relevant social scientific literature and situate their initial assumptions within that corpus. Before expounding on how humans ought to live, the writer must first understand how they do live.

Rawls and Nussbaum have offered their own contributions to this deeply complex project to ground political philosophy in assumptions of human nature. While both fall short of the goal of complete success, the implications of that shortfall differ for their respective ethical systems.

Nussbaum’s failure to satisfy the criteria of both external validity and internal consistency arise from her failure to include necessary arguments to defend her positions (e.g., that it is reasonable to insist that human beings are basically benevolent rather than self-interested) or to accept the implications of those positions even when they contradict her other intuitive preferences, such as an opposition to world government. The limitations within the work of Rawls, however, strike the reader as being contained within the structure of this theory itself. Both his unconvincing psychology within the original position that requires ignorance of and interest in the self, and confusion as to whether the conviction that fellow citizens are free and equal is an initial intuition or a learned social belief, each claim—the thesis and its antithesis—are critical at different points within his justice as fairness argument. These are not gaps in the discussion such as Nussbaum commits, but rather serious tensions within Rawls’s conceptualization itself. While Nussbaum’s shortcomings are thus of the quality of an error, Rawls’s are a problem that may or may not be remediable without a significant reconceptualization of his larger project.

The present review has shown that Nussbaum’s work contains promissory notes on matters that should have been more completely elaborated before asking her readers to commit to her vision of political justice. These missing links in her argument are all the more surprising in light of the “gross flood of words appearing under her name” (Harpham 2002 p. 52). While it may be the case that given time she can fill in the gaps, it is also possible that fuller details on these fundamental issues will force significant rethinking about later features of her scheme. In this sense Nussbaum has rushed prematurely into print with her account, but supporters can still hope that when the full argument is finally available it will deliver the hoped-for impact.

In contrast the present discussion has identified no gaps within Rawls, but instead genuine tensions. Again, it may be the case that further work can explain why these seeming inconsistencies are more apparent than real, promising real work for philosophers, as compared to the imagining in which one might indulge to anticipate Nussbaum’s next move—the difference, perhaps, between the need for a literary analysis of a Dickens novel versus the speculation as to the contents of the final Harry Potter volume. Rawls’s offering therefore emerges as a more mature philosophical text that rises or falls upon our understanding of what he has actually written within its pages, without recourse to unpublished materials that may or may not satisfactorily address the questions.

While the Nussbaum’s capabilities approach may one day eclipse the justice as fairness model of Rawls, today is not that day.