During the seventeenth century, natural law theory became institutionalised as the leading approach to moral philosophy in Protestant Europe. Hugo Grotius (1583–1645) and Samuel Pufendorf (1632–94) were undoubtedly the most influential natural law theorists of the period. Their books went through hundreds of editions and were translated into most European languages. Grotius was widely praised for having been ‘the first to have systematised a science that, prior to him, was nothing but confusion … and impenetrable darkness’. His systematisation of natural law – the science in question – did not consist in deducing moral rules from uncontested definitions and indisputable axioms. His published writings on natural law took no such ‘geometric’ approach. Rather, in a move mirroring Hans Kelsen’s Reine Rechtslehre, Grotius purportedly pioneered a ‘pure’ science of natural law by sharply distinguishing it from divine positive law (i.e. revelation), civil (i.e. Roman) law, and the voluntary law of nations.

In her famous critique of various ‘modern’ presuppositions in moral philosophy, Elizabeth Anscombe suggests that philosophers ought to give up on the idea that there is a sense of ‘ought’ or ‘obligation’ that is specifically moral. This concept, she argues, originally derived its sense from a notion of an authoritative divine law against which human actions could be measured; the presumed existence of such a law made it possible to render absolute verdicts on human acts as licit or illicit. In the absence of recourse to such a law, the terms ‘moral obligation’, ‘moral ought’ and ‘morally wrong’ are empty and best abandoned. It would be better if we look for the ground of normative claims regarding human life and action in our knowledge of our own ‘species’, where this is understood, ‘from the point of view of the activity of thought and choice in regard to the various departments of life’.

Since at least the time of St Clement of Alexandria (150–215), natural law has been a fundamental concept in Catholic accounts of private and public morality. But what is natural about this law (an ontological question) and how we understand it (the epistemological question) has been far from stable. In the twentieth century, a number of competing theories became fashionable, and disagreements among their proponents fuelled an academic growth industry. Underpinning every account of natural law is a presupposition about how faith and reason are related, and a presupposition about how nature and grace are related. Depending on how one construes these primary relationships, one can come up with different accounts of natural law all purporting to be Catholic. There are commonly three different ways of reading the relationship between the couplets ‘nature and grace’ and ‘faith and reason’. These are often described in academic short-hand as Neo-Thomist, Transcendental Thomist and Neo-Augustinian or Augustinian-Thomist.

This chapter will introduce the basic, theoretical architecture of competing Islamic natural law theories from the pre-modern period (ninth to fourteenth centuries). Specifically, it will outline juristic debates in the usul al-fiqh genre on reason as a source of law, where revelation is silent. Thereafter it will reflect on a range of doctrinal debates in which many of those same pre-modern jurists came to a legal determination without reference to scriptural (or any other) texts. Drawing on a curious heuristic they labelled huquq Allah and huquq al-ʿibad (the claims of God and the claims of individuals), I will show that despite not invoking (expressly or otherwise) any natural law account of Islamic law, jurists nonetheless developed law based on a mode of rationality that could be called anything from ‘rational’ to ‘common-sense’ to ‘pragmatic’.

Natural law ethics is a normative theory, which, as its name implies, centres on two key notions: nature and law. It is animated by the idea that nature, and human nature in particular, is the source and ground of the moral laws (or, more widely, moral norms) which govern our nature. Historically, the ‘nature’ component was first theorised in Ancient Greece and Rome, where philosophers argued that human beings are intrinsically directed to and fulfilled by certain ends – the claim of natural teleology. The ‘law’ component found its most pronounced embodiment far earlier, in the scriptures of Ancient Israel, which proclaim a binding set of moral commandments that issue from a transcendent deity. The history of natural law ethics is, put broadly, a mediation between these two cultural inheritances – and is therefore the site of several recurrent controversies. How, exactly, are moral norms embedded in nature?

To examine the prospects for natural law ethics, we need first a working definition of it. But as the previous chapters suggest, any such definition is controversial. Thomas Aquinas defines law as ‘an ordinance of reason directed towards the common good from him who has care of the community and promulgated’. It follows that for a law to be natural, it must be not only grounded in human nature – something Aquinas affirms elsewhere – but also promulgated: the only available promulgator being God. Rosalind Hursthouse, by contrast, takes natural law ethics to be metaphysically less demanding. For her, its theistic underpinning is not salient, and appears even moot. What is salient is the provision of a naturistically grounded criterion of right or good action. Natural law ethics is thus distinct from ethical naturalism in general, which ‘provides a criterion for a particular trait’s being a virtue, not a criterion of right or good action, except indirectly’.

Of all post-Enlightenment social sciences, economics might be easily classified as having the least in common with natural law theory. To the extent, for example, that economics is concerned with matters such as utility maximisation, cost–benefit analysis or identifying unintended consequences of particular decisions, its goals differ from natural law theory’s central focus on individuals and communities (including the state) knowing and choosing the good through right reason. Typically, what is called ‘positive economics’ – famously defined by the British economist Lionel Robbins as ‘the science which studies human behaviour as a relationship between ends and scarce means which have alternative uses’ (Robbins 1952: 16) – brackets off the type of normative questions with which natural law theory is concerned.

Those engaged in natural law reflection have, however, always brought their methods of reasoning to bear upon questions with significant economic components.

Not every creature has reason to live so as to fulfil its purpose in life. Just ask a battery hen. Her purpose in life is to produce as many eggs at as little cost as possible, then be turned into a raw material of the fast-food industry. The resultant misery and distress to the hen is just a corollary of her living according to her purpose, her telos – as a frank egg-farmer might admit. From the battery hen’s own point of view, there is no reason at all for her to live this way. As she would see it (if she had a chance to consider the question), she would be better off doing almost anything other than fulfilling her telos. Her telos and her flourishing are not the same thing. Almost all of the time, they do not even coincide.

The New Natural Law (NNL) theory is the name given to a particular revival and development of Thomistic natural law theory, first proposed in the 1960s by Germain Grisez in an interpretative article on St Thomas Aquinas, in which Grisez challenged the then-dominant interpretation of Aquinas on natural law. In subsequent decades Grisez, John Finnis, Joseph Boyle and others richly developed the theory and applied it to other issues (free choice, moral absolutes, abortion, euthanasia, marriage and others).

According to the interpretation of Aquinas’ natural law theory standard in the 1960s, the basic moral criterion is human nature itself. Actions are morally permissible if they conform to the teleologies inscribed within human nature, morally wrong if not. Since the standard for what is morally right is what fulfils human nature, one must first determine by a theoretical procedure what end or ends fulfil human nature.

The natural law conception of ethics is more closely associated with Thomas Aquinas than with any other thinker, and Aquinas’ position is famously grounded in a metaphysics of Aristotelian essentialism and teleology. It is often claimed that this metaphysics was decisively undermined by the scientific revolution. Hence it might appear that modern science has refuted what has been, historically, the main approach to understanding natural law.

Alternative versions of natural law may seem to sidestep this difficulty by looking for foundations in something other than an essentialist and teleological conception of nature. For example, Locke’s theory of natural law is grounded in the thesis of divine ownership of human beings. Violating the rights to life, liberty and property that we have under natural law is, for Locke, essentially a matter of damaging God’s property.

Any attempt to offer an account of natural law in Stoicism is confronted with the notorious problem of evidence. Not a single work of the ‘early’ Stoics (Zeno of Citium, Cleanthes and Chrysippus, all working in the third century BCE) has fully survived from antiquity. Reconstructions of their views depend on reports by authors who wrote much later and are in many cases anything but unbiased. The information that these authors provide usually leaves a rather wide scope for different interpretations. And the decision between these interpretations is often a matter of the general assumptions which guide our approach to the Stoics: whether we tend to think, for instance, that the early Stoics stood on common ground with their predecessors and contemporaries or whether we assume that they tried to distinguish themselves from other philosophers. As we shall see, this alternative is particularly relevant in the case of the natural law.

By acknowledging the place of reason in bioethics, natural law theory promises an ethics of life and death which, whilst acknowledging the social significance of consensus, consequences, cultural practices and competing religious precepts, offers a school of thought not reducible to these shifting, arbitrary and contradictory criteria. Advances in technology suggest that natural law theory can assist in the conduct of bioethics, if only because the natural law tradition entertains the possibility of universality, reason, objectivity and right answers in the realm of bioethics.

There are schools of thought that regard the death penalty a sound punishment for a change of religion or causing religious offence. In thirteen countries such legislation is recognised as binding and enforceable. These mandates are also seen by millions, both clerics and faith communities alike, as enforceable outside the jurisdictions that enact them.

The account of the natural law given by Thomas Aquinas has two claims to the title ‘natural’. First, the basic precepts of the natural law are naturally known, that is, they are accessible to us by the nature of our human minds. Second, the human good depends upon our nature. Philosophical inquiry, for instance, is part of the human good because of the nature of our minds. If we had the minds of squirrels, then philosophical inquiry would be alien to the human good.

For Aquinas, these two aspects of the natural law are intimately linked by way of natural inclinations. To possess a true nature – and not merely some abstract mathematical ‘nature’ – is to move towards certain ends. This natural movement or impetus Aquinas calls a ‘natural inclination’, not to be confused with an inborn desire.