The manifest appearance of function and purpose in living systems is responsible for the prevalence of apparently teleological explanations of organismic structure and behavior in biology. Although the attribution of function and purpose to living systems is an ancient practice, teleological notions are largely considered ineliminable from modern biological sciences, such as evolutionary biology, genetics, medicine, ethology, and psychiatry, because they play an important explanatory role.

Historical and recent examples of teleological claims include the following:

The chief function of the heart is the transmission and pumping of the blood through the arteries to the extremities of the body. (Harvey 1616 [1928: 49])

The Predator Detection hypothesis remains the strongest candidate for the function of stotting [by gazelles]. (Caro 1986: 663)

The geographic range of human malaria is much wider than the range of the sickle-cell gene. As it happens, other antimalarial genes take over the protective function of the sickle-cell gene in … other warm parts. (Diamond 1994: 83)

Despite the substantial amount of data we now have on theropod dinosaurs, more information is necessary in order to determine the likelihood that early feathers served an adaptive function in visual display as opposed to other proposed adaptive functions such as thermoregulation. (Dimond et al. 2011: 62)

The ubiquity of claims such as these raises the question: how should apparently teleological notions in biology be understood?

Most post-Darwinian approaches attempt to naturalize teleology in biology, in opposition to nineteenth-century viewpoints which grounded it theologically. Nevertheless, biologists and philosophers have continued to question the legitimacy of teleological notions in biology. For instance, Ernst Mayr (1988), identified four reasons why teleological notions remain controversial in biology, namely that they are:

1. vitalistic (positing some special ‘life-force’);
2. requiring backwards causation (because goal-directed explanations seem to use future outcomes to explain present traits);
3. incompatible with mechanistic explanation (because of 1 and 2);
4. mentalistic (attributing the action of mind where there is none).

A fifth complaint is that they are not empirically testable (Allen & Bekoff 1995). The current philosophical literature offers both Darwinian and non-Darwinian accounts of teleology in biology that aim to avoid these concerns. In this article, we hope to bring some clarity to the contemporary debates over the role of teleological notions in biology by sketching a taxonomy of the various accounts of biological function on offer (see Allen & Bekoff 1995 for a more comprehensive taxonomy that forms the basis of this presentation). We primarily focus on naturalistic accounts of biological function, since this is where we see the most lively and productive current debates (see, e.g., Garson 2016 for an extended survey). We also briefly discuss the notion of goal-directedness in section 2.

The idea of the social contract goes back at least to Protagoras and Epicurus. In its recognizably modern form, however, the idea is revived by Thomas Hobbes and was later developed, in different ways, by John Locke, Jean-Jacques Rousseau, and Immanuel Kant. After Kant, the idea fell out of favor with political philosophers until it was resurrected by John Rawls. It is now at the heart of the work of a number of moral and political philosophers.

The basic idea seems simple: in some way, the agreement of all individuals subject to collectively enforced social arrangements shows that those arrangements have some normative property (they are legitimate, just, obligating, etc.). Even this basic idea, though, is anything but simple, and even this abstract rendering is objectionable in many ways.

Political theorists propose standards that identify legitimate uses of political power. Some adopt a principle of public justification. A public justification is a kind of rationale for exercising power and control. It is public because the rationale, or group of rationales, is one that members of the public can accept. Put another way, to treat people as equals, we must ensure that political power is justified for or to them by their own lights, so a public justification thus consists of reasons the public can recognize as valid. Those who adopt a public justification standard are often called public reason liberals. Liberal institutions (freedom of speech, the rule of law, democracy) are publicly justified, but illiberal institutions are not. Diverse perspectives within the public will reject non-liberal institutions.

Coercion is the standard object of public justification because it is perhaps the characteristic feature of political life. Charles Larmore remarks that public justification has “to do with the sort of respect we owe one another in the political realm — that is, in relationships where the possibility of coercion is involved” (Larmore 2008, 86). John Rawls’s principle of public justification holds that political power requires justification (Rawls 2005, 12) because “political power is always coercive power” (Rawls 2005, 68). Jonathan Quong holds that public justification concerns the imposition of coercive laws (Quong 2011, 233–250). And, as Christopher Eberle puts it (2002, 54), “the clarion call of justificatory liberalism is the public justification of coercion.” Some have wondered whether non-coercive state actions need public justification (2.7). But they nonetheless agree that coercion generally, if not always, requires it.

The idea of a public justification is, at its root, an idea about which reasons justify coercion. Public justification is not a process of exchanging reasons. Instead, the exchange of reasons can uncover or generate a public justification. Or we could arrive at a public justification through a non-deliberative route. Examples include bargaining processes and adjudicative procedures. In this way, the ideas of public reason and public justification are distinct. Public justifications might consist of public reasons. But a reason shared by the public might fall short of a sound justification if other reasons undercut or override it. One might, for example, support a new anti-poverty program on shared grounds of justice for the poor. However, alternative programs reduce poverty more effectively. The original poverty program could be publicly justified based on a shared commitment to justice, but other shared reasons, like policy efficacy, can undercut the public justification for the program.

Rawls was the foremost advocate of the idea of public justification. But we find the idea stressed in the works of Jürgen Habermas, David Gauthier, Gerald Gaus, Stephen Macedo, Charles Larmore, Seyla Benhabib, and many others.

There is considerable disagreement about how to understand the idea. Some theorists hold that all public justifications consist of shared or accessible reasons. These are often called consensus theorists. Others allow diverse, unshared reasons to figure into public justifications. These are often called convergence theorists. (See Section 2.3 below). Public justification theorists also disagree about how to attribute reasons to citizens. This disagreement is about the right level of idealization. Idealization involves modeling someone as having improved information and cognitive capacities. The goal is to identify which reasons apply to her, even if she cannot or will not see them as such in her ordinary life. Some theorists adopt more radical idealizations than others.

This entry addresses disputes about public justification by articulating an open-ended principle. This Public Justification Principle (PJP) helps classify competing conceptions of public justification.