While most things seem to go back to Aristotle, Jean Porter is keen that for once we stop short, and consider Saint Thomas Aquinas's work on natural law not as a Christian interpretation of Aristotle, but rather as a product of mediaeval scholastic thought. Nature as Reason builds on Porter's 1999 Natural and Divine Law, a study of the scholastic concept of natural law developed in the twelfth and thirteenth centuries. This new book develops a constructive theory of the natural law for the modern world, developed through reflective interpretation, taking Aquinas and his contemporaries as conversation partners, with whom to think through central issues. This is not simply a book about Aquinas's relevance today, both because Aquinas is only one of the scholastics with whom Porter engages, and also because Porter doubts that Aquinas would agree with all her conclusions.

Porter suggests that scholastic natural law theory is worth bothering with today because it provides us, subject to some adjustments, with a fundamentally Christian way of doing ethics. This recognises our place as moral agents within God's creation. It is intrinsic to the character of the human soul as made in the image of God that we possess an innate capacity to distinguish good from evil. Humans, like all other creatures, possess the capacity and inclination to fulfil their natures. What Porter's approach expressly does not permit us to do is to extract fundamental principles of natural law simply from observing the natural world, or to produce single right answers from first principles. Ethics allows us to reason through our moral decisions for ourselves, but it does not dictate answers.

The cornerstone of Porter's thesis is her argument for the naturalness of human nature. Natural law, she says, should be understood both in terms of Reason and in terms of Nature. Human nature is itself morally significant, both because we are joined to the rest of a self-realising creation, and also because we are distinct from the rest of creation by virtue of being made in the image of God. This concept of nature requires that species have a real existence beyond the particular creatures that instantiate them. Relying on the work of a number of philosophers of biology, Porter argues that creatures can be seen as falling into an ordered and hierarchical set of natural kinds. When creatures evolve, they seem to do so with a sense of purpose towards the flourishing of their natural kind, which we cannot explain purely in terms of random mutation and selection. Thus, when finches evolve they do so to flourish in their ‘finch-ness’ and not to become rhesus monkeys or mushrooms. The fact that we may have more difficulty defining ‘human-ness’ than ‘finch-ness’ does not mean that there is no such thing as an essential ‘human-ness’, says Porter, but only that our attempts to define it fully are limited by our historical contingency.

Porter's argument on essential natures is attractive, but it does not engage fully with possible counter-arguments. First, the tidiness of species taxonomies might be explicable by the evolutionary theory of common descent. Secondly, while evolving finches may maintain their ‘finch-ness’ within historical time, we have reason to believe that within geological time, finches evolved out of reptiles. If a robust theory of an essential human nature cannot be provided, then one of Porter's other significant contributions, a critique of the ‘new theory of natural law’ is significantly weakened.

The ‘new theory of natural law’ was an attempt by German Grisez and John Finnis in the 1980s and 1990s to overcome the detailed critiques of traditional natural law theory that had been developed since the eighteenth century. They suggest that we are directed by practical reason towards certain basic goods, and that these basic goods are the only intelligible reasons for action. This theory appears to fall firmly within the Kantian tradition, according to which moral norms are grounded in the deliverances of practical reason itself. Practical reason therefore functions independently of the agent's emotions and desires. At least in part, the success of the new natural law beyond the traditional realm of Catholic moral theology has been because one might plausibly argue that the basic goods can be identified and pursued without reference to God. This atheistic argument is presumably not one with which Grisez and Finnis would agree.

But new natural law theory is flawed, argues Porter, since Kantian and consequentialist ethics, such as utilitarianism, are fundamentally unstable. They tend to collapse into one another unless we inject into them speculative considerations about such matters as the metaphysical status of the human person. Not surprisingly, this is solved by (what is for Porter) a correct reading of Aquinas: our actions arise from our desires and commitments, while the principles of rationality tell us how to achieve these consistently. These desires and commitments are correctly shaped by a proper understanding of our human nature, and are themselves subject to rational evaluation. Similarly, Porter suggests that modern human rights law lacks proper moral justification, because its proponents shy away from explaining why they believe that people possess inalienable rights.

Porter's view of the natural law is radical, both in the sense that she claims to take us back to an authentic reading of natural law's scholastic roots, and in the sense that she departs from the approach of modern Catholic ethics, which has often sought to derive single right answers from the natural law. For Porter, the question moves from ‘what does reason dictate?’ to ‘what most enables us to flourish truly as human beings?’ The detail of Porter's natural law theory is developed over 400 closely argued pages, in which she successfully moves back and forth between the contributions and challenges of both mediaeval and modern philosophers and theologians in this field. Against this significant achievement, there are minor points that might usefully be addressed, such as possible counter-objections to Porter's philosophy of biology, mentioned above, and the delayed arrival of a detailed theological discussion until the final chapter. Nature as Reason represents a very valuable contribution both to natural law theory and to moral theology, and Porter's argument deserves to be taken seriously.