What Is the Naturalistic Fallacy?

Direct Answer

The naturalistic fallacy is a philosophical concept identified by G.E. Moore in his seminal work Principia Ethica (1903). It is the error of defining moral properties — particularly the property of "good" — in terms of natural or empirical properties such as pleasure, happiness, desire-satisfaction, or evolutionary fitness. Moore argued that "good" is a simple, non-natural property: it cannot be broken down into or reduced to anything else, any more than the color yellow can be defined in terms that do not presuppose the experience of yellow itself. To attempt to define good in natural terms is to commit a fallacy — to mistake a moral property for a natural one.

Moore's argument for this claim is the famous Open Question Argument. The argument runs as follows: take any proposed natural definition of good — say, "good means that which produces pleasure." Now ask the question: "Is that which produces pleasure good?" If the definition is correct, this question should be closed — it should be analytically true that what produces pleasure is good, just as it is analytically true that a bachelor is unmarried. But the question is not closed. It remains perfectly meaningful to ask whether what produces pleasure is really good — we can coherently wonder whether pleasure is the right thing to pursue, whether some pleasures might be bad, whether there might be goods that are not pleasurable. The fact that this question remains open, Moore argues, proves that "good" and "pleasure" are not identical. What is true of pleasure is true of every proposed natural definition: the question of whether the natural property is good always remains open, and therefore good can never be reduced to any natural property.

The naturalistic fallacy is related to, but distinct from, the is-ought problem identified by Hume. The is-ought problem concerns the logical derivation of normative conclusions from descriptive premises: you cannot validly infer "one ought to do X" from premises that merely state what is the case. The naturalistic fallacy concerns the definition of moral terms: you cannot define "good" in terms of natural properties. Both identify a gap between the natural and the normative, but they do so from different directions — the is-ought problem concerns reasoning, while the naturalistic fallacy concerns semantics and metaphysics. Together, they constitute the foundational challenge to any philosophy that attempts to ground morality in nature alone.

Historical Context

The naturalistic fallacy emerged in the context of late Victorian moral philosophy, which was dominated by two approaches that Moore sought to challenge. The first was utilitarianism, particularly the hedonistic utilitarianism of Henry Sidgwick, which defined the good as pleasure or happiness and held that the right action is the one that maximizes this good. The second was evolutionary ethics, associated with thinkers like Herbert Spencer, which attempted to derive moral values from the evolutionary process — arguing that what is "more evolved" is thereby better, or that the moral good is whatever promotes the survival and flourishing of the species.

Both approaches commit what Moore would call the naturalistic fallacy: they define "good" in terms of a natural property (pleasure, in the case of utilitarianism; evolutionary fitness, in the case of evolutionary ethics). Moore's argument was that these definitions are not merely incorrect but meaningless — they fail to capture what moral language actually means. When we say that something is good, we do not mean that it is pleasurable or that it promotes survival; we mean that it has a distinctive moral quality that is not reducible to any natural property. The utilitarian and the evolutionary ethicist are not offering different answers to the same question; they are answering a different question than the one that moral philosophy should be asking.

Moore's work was part of a broader intellectual movement — the revolt against idealism that characterized British philosophy at the turn of the 20th century. The dominant philosophical tradition in Britain at the time was absolute idealism, associated with T.H. Green and F.H. Bradley, which held that reality is ultimately spiritual or mental and that moral values are grounded in the absolute. Moore and his Cambridge contemporaries, particularly Bertrand Russell, rejected this idealist framework in favor of a realist metaphysics that took the natural world at face value. But Moore also rejected the naturalistic ethics that some realists embraced: he argued that while the natural world is real, moral properties are not natural properties and cannot be reduced to them.

The concept of the naturalistic fallacy was also shaped by Moore's engagement with Aristotle and the tradition of virtue ethics. Aristotle had defined the good as eudaimonia — flourishing or well-being — and this definition has seemed to many philosophers to be a naturalistic one, identifying the good with a natural condition of the human being. Moore's argument challenges this identification: even if eudaimonia is the good, it is not identical to the good by definition, because one can coherently ask whether eudaimonia is really good. The naturalistic fallacy thus raised questions not only for utilitarianism and evolutionary ethics but for the entire Aristotelian tradition of defining the moral good in terms of human nature.

Philosophical Perspectives

Moore's Open Question Argument

The Open Question Argument is the philosophical heart of Moore's case against naturalism. Its structure is deceptively simple: for any proposed naturalistic definition of good, we can always meaningfully ask whether the natural property so defined is actually good. If "good" meant "pleasurable," then asking "Is pleasure good?" would be like asking "Is a bachelor unmarried?" — a closed question whose answer follows analytically from the definition. But "Is pleasure good?" is not a closed question; it is an open one, a genuine question that thoughtful people can and do disagree about. This openness, Moore argues, demonstrates that "good" and "pleasure" are not synonymous — that the proposed definition fails.

The power of the Open Question Argument lies in its generality. It applies not just to pleasure but to any natural property that one might propose as the definition of good. Is desire-satisfaction good? Is survival good? Is the flourishing of the species good? Each of these remains an open question, and each openness reveals that the proposed natural property is not identical to the moral property of goodness. Moore concluded that "good" is indefinable — not because we cannot know what things are good, but because the concept of good cannot be reduced to any other concept. It is a simple, unanalyzable property, like yellow or round, that must be grasped directly rather than defined in terms of something else.

Moore's account of good as a non-natural property has important metaphysical implications. If good is a real property but not a natural one, then reality contains not only the natural properties studied by science but also non-natural moral properties that are apprehended not by empirical observation but by moral intuition. This is Moore's moral intuitionism: the view that we can directly apprehend moral truths through a faculty of moral intuition, just as we directly apprehend colors through visual perception. Moral intuitionism was the dominant view in British moral philosophy for the first half of the 20th century, and it was Moore's naturalistic fallacy that gave it its philosophical foundation.

Hume's Is-Ought Problem

The naturalistic fallary is closely connected to the is-ought problem that David Hume identified in the 18th century. In A Treatise of Human Nature, Hume observed that philosophers often move imperceptibly from descriptive claims about what is the case to normative claims about what ought to be the case, without explaining how the transition is made. This transition, Hume argued, is illegitimate: no ought-statement can be logically derived from is-statements alone, because the logical relation between descriptive and normative claims is not one of entailment.

The is-ought problem and the naturalistic fallacy identify the same gap from different directions. Hume's argument concerns reasoning: you cannot infer an ought from an is. Moore's argument concerns definition: you cannot define an ought (or a good) in terms of an is. Both rest on the insight that the normative and the descriptive are logically distinct — that there is a gap between statements about what the world is like and statements about what should be done or what is valuable — that cannot be bridged by definition or by deduction alone. This gap is one of the most important discoveries in the history of moral philosophy, and it continues to shape contemporary debates about the foundations of ethics.

The relationship between the two arguments is not merely parallel but complementary. Hume's is-ought problem establishes the logical gap between facts and values; Moore's naturalistic fallacy establishes the semantic gap between natural and moral concepts. Together, they constitute a powerful challenge to any attempt to ground morality in nature — whether that grounding takes the form of deriving moral conclusions from natural facts (the Humean challenge) or defining moral properties in terms of natural properties (the Moorean challenge). Any naturalistic ethics must address both challenges if it is to be philosophically defensible.

Challenges to the Naturalistic Fallacy

Despite its enormous influence, the naturalistic fallacy has been subject to serious philosophical criticism. The most fundamental challenge comes from the observation that the Open Question Argument may prove too much. If the openness of the question "Is pleasure good?" proves that "good" and "pleasure" are not identical, then by the same logic, the openness of the question "Is water H2O?" would prove that "water" and "H2O" are not identical. But we know, since the chemical revolution, that water is H2O — the identity is a scientific discovery, not an analytic truth. The fact that a question is open does not prove that the terms it contains are not identical; it may merely prove that the identity is synthetic rather than analytic — a substantive discovery rather than a matter of definition.

This challenge, developed by philosophers like Frank Jackson and Philip Pettit, suggests that the Open Question Argument conflates analytic and synthetic identity. Moore assumed that if "good" were identical to some natural property, the identity would be analytic — knowable from the meanings of the terms alone. But identities can be synthetic — discoverable only through empirical or philosophical investigation. If "good" is identical to some natural property (say, the property of promoting flourishing), this identity might be synthetic rather than analytic, and the Open Question Argument would not refute it. This "synthetic naturalism" remains a live option in contemporary meta-ethics.

Another challenge comes from Cornell realism, associated with philosophers like Richard Boyd and Nicholas Sturgeon, which argues that moral properties are natural properties — not reducible to non-moral natural properties in the way that Moore assumed, but identical with certain higher-level natural properties that are discovered through moral inquiry rather than through empirical science. The Cornell realist argues that moral properties are like biological properties: they are natural but not reducible to physics, and they are discovered through a specific mode of inquiry rather than through definition or observation alone. This approach rejects the naturalistic fallacy by rejecting the premise that natural properties must be reducible to physical properties — a premise that Moore took for granted but that contemporary philosophy of science has shown to be questionable.

Modern Reflection

Contemporary philosophy continues to debate the naturalistic fallacy with intensity, and the debate has been enriched by developments in several areas. The rise of meta-ethics as a distinct philosophical discipline — devoted to questions about the nature and status of moral claims rather than to first-order moral questions — can be traced directly to Moore's work. The central questions of meta-ethics — Are moral properties natural or non-natural? Are moral judgments cognitive or non-cognitive? Can moral claims be true or false? — are all shaped by the framework that Moore established, even when they reject his specific conclusions.

The development of expressivism and non-cognitivism in the 20th century can be seen as one response to the naturalistic fallacy. If moral properties are not natural properties (as Moore argued), and if non-natural properties are metaphysically suspect (as many 20th-century philosophers believed), then perhaps moral language does not refer to properties at all. The emotivists — Ayer and Stevenson — argued that moral judgments are not assertions about properties but expressions of attitude. The prescriptivists — notably R.M. Hare — argued that moral judgments are not descriptions but prescriptions, commands disguised as assertions. These views accept Moore's rejection of naturalism but draw a different conclusion: not that moral properties are non-natural, but that there are no moral properties at all.

The contemporary revival of moral naturalism represents the other main response to the naturalistic fallacy. Philosophers like Peter Railton, David Brink, and the Cornell realists have developed sophisticated forms of naturalism that attempt to meet Moore's challenge head-on. These approaches argue that moral properties are natural properties — not in the reductive way that Moore criticized, but in a more subtle way that draws on the philosophy of science's understanding of how higher-level properties relate to lower-level ones. The debate between naturalists and non-naturalists remains one of the liveliest in contemporary meta-ethics, and it is conducted largely within the framework that Moore established.

The practical implications of the naturalistic fallacy extend beyond academic philosophy. In bioethics, the distinction between natural and normative claims is central to debates about enhancement, disability, and the definition of health. In environmental ethics, the question of whether one can derive ecological values from ecological facts turns on the is-ought distinction that Hume identified and Moore developed. In public policy, the temptation to derive ought from is — to move directly from scientific findings about what is the case to policy recommendations about what should be done — is ever-present, and awareness of the naturalistic fallacy provides an important check on the uncritical conflation of the descriptive and the normative.

The question of whether the naturalistic fallacy is genuinely a fallacy — whether the move from natural properties to moral properties is truly an error — remains open. Moore's argument has been challenged on multiple fronts, and many contemporary philosophers believe that some form of naturalism is defensible. But even those who reject Moore's specific conclusion acknowledge that he identified something important: the gap between the natural and the normative that any adequate moral philosophy must address. Whether that gap is bridged by non-natural moral properties, by a non-cognitive account of moral language, by a sophisticated naturalism, or by some other means, the naturalistic fallacy remains the starting point for one of the most important conversations in philosophy.

Related Thinkers

• Aristotle — the ancient philosopher whose definition of the good as eudaimonia represents one of the targets of Moore's argument, since it appears to define the moral good in terms of a natural condition of the human being, and whose virtue ethics tradition has been central to debates about whether moral properties can be naturalized.
• Hume — the philosopher whose is-ought problem identified the logical gap between descriptive and normative claims that complements Moore's semantic gap between natural and moral properties, and whose empiricist approach to moral philosophy shaped the tradition that Moore both inherited and challenged.

Related Books

• Principia Ethica by G.E. Moore — the foundational text of meta-ethics, presenting the naturalistic fallacy, the Open Question Argument, and the case for moral intuitionism that shaped twentieth-century moral philosophy.
• A Treatise of Human Nature by David Hume — the work in which Hume identified the is-ought problem, the logical counterpart to Moore's naturalistic fallacy, and established the distinction between descriptive and normative claims that remains foundational to contemporary meta-ethics.

Related Quotes

• "Good is a simple notion, just as yellow is a simple notion; just as you cannot, by any means, explain to anyone who does not already know it, what yellow is, so you cannot explain what good is." — G.E. Moore
• "Every ought implies a can, but no is implies an ought." — Immanuel Kant (reflecting the principle that underlies both Hume's is-ought problem and Moore's naturalistic fallacy)
• "The transition from is to ought cannot be made without adding a new premise." — David Hume (paraphrased)
• "If I am asked, What is good? my answer is that good is good, and that is the end of the matter." — G.E. Moore

Related Topics

• Knowledge and Truth — The naturalistic fallacy raises fundamental questions about the nature of moral knowledge: if moral properties are non-natural, how can we know them, and what is the relationship between moral truth and empirical truth?

Further Learning

To deepen your understanding of the naturalistic fallacy, explore these connected resources in the ZHAIBIAN archive:

• Truth — the wisdom entry on the nature of truth, within which the question of whether moral judgments can be true or false is one of the central issues raised by the naturalistic fallacy
• Morality — the wisdom entry on the nature of moral judgment and moral agency, within which the naturalistic fallacy raises the question of whether moral properties are natural or non-natural
• Epistemology — the philosophy entry on the theory of knowledge, within which the question of how we can know moral truths — if they are non-natural — is one of the challenges that Moore's argument poses
• Virtue Ethics — the philosophy entry on the tradition that defines the good in terms of human flourishing, a definition that the naturalistic fallacy challenges as an attempt to reduce the moral to the natural
• Understanding Reality — the curated collection on how philosophy, science, and wisdom traditions approach the question of what is real, within which the debate about whether moral properties are real and natural is a central theme