Contemporary theories of justice are about, in Rawls’ words, ‘the basic structure of society’, and Jean Porter begins her magisterial study of Aquinas on justice by noting he does not offer a theory of justice in this sense. Rather, Aquinas understands justice as a ‘personal virtue’, and in particular ‘a virtue of the will’, involving a ‘stable disposition that is grounded in an abstract, reasoned conception of the good’, operating to ‘integrate the agent's passions with her reasoned convictions and overall aims’ (3), convictions and aims which, given the kinds of creatures we are, naturally, though not necessarily, must be conceived in relation to the goods of others and the whole community. ‘Aquinas gets justice right’ (p. 5), Porter claims, and in doing so, offers ‘the integrating key for a comprehensive account of moral value, seen as both grounded in, and yet qualitatively different from, the aims and values natural to us as living creatures of a certain kind’ (p. 4). Thus, an important aim of her study is to show that an account of justice as a virtue overcomes dichotomies between nature and reason, emotions and rationality, and legal and eudaimonistic virtue-centred accounts of morality.

The key is understanding Aquinas’ claim that virtues are ‘perfections’ of specific human capacities, and justice is the virtue that perfects the will. The will is a special kind of appetite for ‘one's overall existence and full development in accordance with some reasoned conception of what it means to live an appropriate or desirable or ideal life’ (p. 71). Justice perfects the will, which is by definition indeterminate, as it aims at the overall good of the agent rationally apprehended. But this overall good is not merely a matter of harmonising internal elements of the soul, but of excellent activity toward and with others. This is because the will is also ‘the principle for actions ad extra’ – that is, the will is at the heart of Aquinas's action theory – which are ‘innately oriented toward others . . . mov[ing] the agent to relate to others in appropriate ways’ (p. 229). This twofold role of the will as rational appetite and as principle of action then shapes Aquinas's account of justice as its specific virtue. The virtue of justice perfects the rational appetite by a maturing and increasingly stable desire for a determinate (though not fixed) conception of a good way of life. While full rectitude of the will ‘is identified with the supernatural virtue of charity’, there is a ‘connatural rectitude, which disposes the will toward happiness, understood as a morally upright life pursued through the practice of the virtues’ (p. 103). Porter stresses that justice is not the only perfection of the will, but nevertheless it is a genuine perfection of the person; importantly, a life of justice need not be contrasted with eudaimonism, though an agent's flourishing also cannot be reduced to a life of justice.

Proceeding from this core moral psychology, later chapters develop justice as a ‘normative ideal’, emphasising the fundamental notion of equality at the core of Aquinas’ treatment of justice in the Summa and the peculiar stringency of the norms associated with it. The ‘complex theory’ that emerges ‘reflects a strong sense of the complexity of our lives as moral agents’ in integrating competing claims, but allows ‘a fuller account of justice as a personal quality of character, a disposition toward others that is admirable and praiseworthy in itself and that is integrally connected to a worthwhile and desirable way of life’ (p. 241). The tight relationship of the agent's own vision of overall well-being with the demands of justice is seen in the fact that the same developmental process fosters both. Engagement with particular projects inductively shapes emerging, stable commitments of the will to a comprehensive good – and is also the process through which we come to recognise and desire the norms of justice relating us to particular others and to the general common good. When tension between self and others still occurs, Aquinas offers a resolution by the ‘further conceptual and volitional leap’ by which the general norms of justice respected toward particular others within one's project are also seen to apply to all others, and he argues that respect for such norms (and thus for other persons) ‘can be stringently binding and overriding, even in the face of conflicts with her most cherished ideals and commitments’ (p. 263).

This exhaustive study not only engages many key arguments (e.g. the meaning of natural law and natural rights), but also provides able readers with impressive summaries of Porter's previous work, including a summary of her earlier Nature as Reason in this book's chapter 1. While the text is demanding, the precision of the final theory, particularly as it unfolds in the final chapter, is well worth the work. Her complex account serves as a real alternative to (though not necessarily rejection of) the quasi-deontology of theories such as the new natural law theory and the quasi-structuralism of typical liberal accounts of justice. Thus, it should be required reading for ethicists across the ideological spectrum. In a time of conflict, Porter attractively stresses the contingency of our ability to develop the virtue of justice. Justice emerges in agents when they learn (a) stable desires for comprehensive goods greater than the self and (b) stable desires to treat others in accord with the particular norms of respect and equality that are rooted in the types of creatures we are. In recognising the contingency of such formation, she illuminates why it is so challenging to move beyond a ‘morality of obligation’ approaches to justice, whether of the left or right. Such approaches neglect the need to develop a stable desire to follow norms. This insight depends on ‘getting justice right’ as a specific virtue perfecting the will. Porter's book does not end up resolving all the divisive issues of the demands of justice, but it explains the framework within which fruitful exchange about these demands should take place.