Depending on how one defines the category, natural law thinking goes back centuries, and perhaps millennia, in the Western tradition. Elements of that tradition can be found in Plato, Aristotle and Sophocles; it was developed in a systematic way by Thomas Aquinas in the thirteenth century. At the same time, natural law can also sometimes seem like the “new, new thing”, fashionable again among a group of young scholars. More evidence of natural law's new-found popularity comes from two books from Cambridge University Press in 2017 – The Cambridge Companion to Natural Law Jurisprudence and Christianity and Natural Law – and there is one more Cambridge volume on the topic scheduled for this year, by Jonathan Crowe, Natural Law and the Nature of Law (Cambridge, 2019). Additionally, the US State Department under the Trump Administration recently announced the formation of an advisory “Commission on Unalienable Rights”, which will stress human rights in the context of natural law. The commission will be chaired by Mary Ann Glendon, and will contain one of the contributors to the Cambridge Companion, Christopher Tollefsen. That commission has already been the subject of widespread criticism, as some commentators believe that natural law talk is just a cover for cultural conservatives to push back against rights for the LGBTQ community and other rights claims associated with progressive views.

As that debate indicates, natural law has problems of perception and identity. On the one hand, theorists within the tradition need to show that natural law is not simply another name for one side of the “culture wars”. On the other hand, there remains a problem of demarcation from other approaches to moral philosophy, as, at its broadest and most abstract, “natural law” appears to be merely another label for belief in “(objective) moral truth”. As Russell Sandberg notes in Christianity and Natural Law: “Many of the ideas associated with natural law theory become much less controversial if they are presented without naming the theory”.

In narrower uses, “natural law” refers to a particular way of thinking about the connection between God, nature, human beings and the right way to live; for some advocates of natural law, the tradition is connected, more specifically, with the approach developed by Aquinas. Norman Doe, the editor of one of these volumes, offers the following summary of the tradition:

“The term ‘natural law’ is used for a variety of doctrines which are often contested and differ in detail. They propose the existence of universal principles derived from nature, particularly human nature, which are normative, providing moral standards or ethical constraints on human behaviour, which determine what actions are right and wrong” (p. 184).

To the extent that the natural law tradition (or traditions) vary from other theories of objective moral truth, it tends to be in the role of divine order or divine plan – connecting human nature teleologically to a role in that plan. A number of the contributors to these volumes also equate natural law theory with a rejection of utilitarianism and other forms of consequentialism (e.g. Duke/George pp. 71–72, 146–48, 229–32, 377–78). It is worth noting that there is no agreement in the tradition on what the significance is of “natural” in “natural law theory”. And the general picture of natural law morality also varies: the views presented in these volumes partake of elements of virtue ethics, deontology and divine command theory, in varying combinations.

The Cambridge Companion to Natural Law Jurisprudence offers first-class contributions from 16 eminent scholars, including some of the best-known figures in the field, including John Finnis, Robert Alexy, Mark Murphy, N.E. Simmonds and Robert George, along with experts in particular subfields: for example, Knud Haakonssen on the history of ideas, Kristen Rundle on Lon Fuller, Verónica Rodriguez-Blanco on practical reasoning and Mark Greenberg on his Moral Impact Theory. Finnis, Alexy, Simmonds and Greenberg all give useful summaries that will introduce readers to ideas these authors have elaborated at greater length elsewhere, while other contributors offer useful interventions on more specific debates: for example, Gerald Bradley on constitutionalism, George Duke on the nature of the common good, Christopher Tollefsen on practical reasoning, and Jacqueline Laing on various topics in applied ethics. Overall, the Cambridge Companion is an excellent resource for study of this school of thought. (A parallel Cambridge Companion to Legal Positivism, edited by Torben Spaak and Patricia Mindus, is expected in the near future.)

Christianity and Natural Law offers an overview of how the idea of natural law is understood in a range of “different Christian traditions or church families worldwide” (p. xiv). While the project of this collection is to show the view of natural law from within various Christian traditions (there is a brief chapter on natural law in Judaism and Islam, though both traditions are presented as being less inviting to natural law thinking, because of their emphases on divine revelation/command), the text reminds us that natural law is generally understood as something applicable to, and subject to understanding by, everyone, even non-believers. The standard quotation from within the Christian tradition (or traditions) is to Romans 2:15: “what the law requires is written on the hearts [of nonbelievers] while their conscience also bears witness.” As the collection shows, there is a great diversity of approaches to natural law across different Christian groupings, and even within particular theological traditions. There are differences about the role of divine revelation, writings of the Church Fathers, human reason, conscience and experience. Several Protestant traditions generally seem wary of the focus on reason and rationality in natural law thinking, preferring approaches more focused on faith and revelation.

Many law students today know natural law only vaguely, as the view against which legal positivism defines itself. However, matters are more complicated. Natural law theorists frequently view the purported dividing lines between natural law theory and legal positivism – most suggested by legal positivists – as both inaccurate and unhelpful. Perhaps the closest thing to a dividing line suggested from the natural law side in these volumes is given in the Introduction to the Cambridge Companion by George Duke and Robert George: that natural law theorists argue for the “impossibility of providing a satisfactory legal theory without reference to wider considerations of practical reason, normativity and justice”.

Crowe's chapter in the Cambridge Companion offers a good overview of the approaches contemporary natural law theories take to law. As Crowe notes, one starting point is that “law is necessarily a rational standard for conduct”. This might mean either that legal rules and legal systems that do not meet this standard are “not law” (the “strong natural law view”), or that those rules or systems that do not meet the standard are “defective law” the (“weak natural law view”). One can then ask how one justifies either of those views; a variety of approaches have been offered: a hermeneutic argument based on our actual social and linguistic practices, an argument grounded on viewing law as a functional kind, or an argument based on seeing law as a kind of speech act or claim (as in Robert Alexy's well-known argument grounded on the “claim to correctness”). Crowe goes on to defend a functional kind argument grounded on law's artefactual nature. In particular, Crowe argues that legal rules serve as deontic markers for citizens’ behaviour. In a separate chapter, Mark Murphy also offers a view of law as an “artefactual kind”, as part of a defence of natural law theory from criticisms offered by various legal positivists (law as artefact was the focus of another recent collection, Law as an Artifact (Luka Burazin, Kenneth Einar Himma and Corrado Roversi (eds.) (2018)).

Verónica Rodriguez-Blanco argues that the “empirical” – social-fact, convention-based or acceptance-based – understandings of law offered by many contemporary legal positivist theories cannot account adequately for citizens’ practical reasoning when they obey the law, or on those occasions they do not (e.g. generally stopping at red lights when driving, but ignoring some red lights when rushing a very sick friend to the hospital). For Rodriguez-Blanco, Hart's “internal point of view” is too internal; it does not adequately account for the external, good(s)-focused, aspects of practical reasoning.

In general, the Cambridge Companion is tilted towards elaborations and defences of the “new natural law theory” (John Finnis prefers the title, “new classical theory of natural law”) associated with the reading of Aquinas offered by Finnis, Germain Grisez and Joseph Boyle, though contrary views are cited and discussed in the volume. By contrast to this Aquinas-based and reason-focused approach, Thomas Pink provides a David Hume-inspired approach that argues that morality is about what is good and admirable, not (just) what is reasonable. Pink also emphasises (contra some Kantian views) that what is admirable morally is not just “the good will” (good motivation), but also the ability to achieve good consequences in the world. At the same time, the analysis avoids the opposite extreme view, that morally good motivation is only relevant to the extent it leads to actions to morally good consequences. Good motivation (will) does matter on its own, Pink argues, for to live with others of good will, whatever the real-world consequences they have been able to effect, is to live in a moral community, where one is treated by others as an object of concern.

N.E. Simmonds distinguishes between “law” as the name for a certain kind of social institution and “law” as a standard or ideal within that social institutional practice. He goes on to explore three puzzles regarding law: (1) If justification in legal practice is generally according to an earlier, more general or more basic legal norm, what justifies the foundational or most basic norm of the system? (2) How can one explain the fact, that while law's content seems grounded on certain sources, like statutes and case decisions, it does not seem to be fully reducible to those sources (such that reasonable, competent practitioners frequently disagree about what the law requires)? and (3) How can the actions of fallible legal officials justify the imposition of obligations? Simmonds argues that many of these puzzles can be resolved if one sees the way that jurisprudential questions about the nature of law – the “idea of law” – often guide the determination of legal content. (A focus on law's “ideal dimension” is also the focus of Robert Alexy's contribution to the Cambridge Companion.) This “idea of law”, or “ideal” of law (pp. 266–67), is tied to traditional ideas about the rule of law (as elaborated in Lon Fuller's eight principles of legality), and is displayed in how judges and scholarly commentators develop legal doctrine. Simmonds concludes that the idea of law is “the idea of a system of rules embodying an evolving but coherent conception of justice”.

Mark Greenberg's contribution to the Cambridge Companion offers further elaborations, clarifications, and arguments to his influential and provocative “Moral Impact Theory”, that “legal obligations are those genuine obligations that obtain in virtue of the actions of legal institutions”. Greenberg's theory responds to certain basic problems in legal practice, including how and why the content of law differs from the text of conventional legal sources (statutes, case decisions, constitutional provisions, etc.). Greenberg's analysis is significant also for the way it takes seriously “how to explain, at the most basic level, the obtaining of legal facts in terms of more basic facts”. In its general structure of analysis, Greenberg's theory may at first glance not look that different from the Aquinas-Finnis view (that positive law consistent with natural law “binds in conscience” – roughly, creates moral obligations) or even David Enoch's view that law can sometimes, but not all the time, “trigger” our existing moral duties. However, under those approaches, legal rules can sometimes – not always – create moral obligations; under Greenberg's approach, law just is the moral obligations created by the actions of legal officials. There is not space here to give appropriate attention to all the interesting arguments Greenberg raises and the issues his approach evokes, though it should be noted, at least, that the Moral Impact Theory may risk circularity (a concern I first heard raised by Larry Alexander): in that it seeks to define “law” as the moral effects of the actions of legal officials.

Christianity and Natural Law offers a valuable overview of the theological battles within Christianity about natural law thinking. For scholars more interested in the nature of law and the role of morality both in legal theory and legal practice, The Cambridge Companion to Natural Law Jurisprudence is the best single-volume introduction currently available.