Defining the Undefinable: G.E. Moore's Naturalistic Fallacy in Meta-Ethics

In his seminal 1903 work *Principia Ethica*, G.E. Moore introduced the concept of the 'Naturalistic Fallacy,' a foundational critique in meta-ethics. The fallacy occurs when a philosopher attempts to define the term 'good' by equating it with a natural property—any quality, such as pleasure, desire, or evolutionary fitness, that can be studied by empirical science. Moore contended that 'good' is a simple, indefinable concept. To equate 'good' with 'pleasure', for example, is not to offer a definition but to make a substantive ethical claim that things possessing pleasure also possess the separate property of goodness. Moore's primary justification is the 'Open-Question Argument.' He argued that for any natural property 'N' proposed as a definition for 'good', the question "Is N good?" always remains a meaningful, or 'open', question. Unlike the closed, tautological question "Is a bachelor an unmarried man?", asking "Is pleasure good?" invites genuine reflection and debate. This demonstrates that 'good' and 'N' are not synonymous. Moore thus concluded that 'good' is a simple, non-natural property that we grasp not through definition or empirical observation, but through a faculty of moral intuition. While his theory of intuitionism has been heavily criticized, his analysis of the naturalistic fallacy profoundly shaped 20th-century ethics by demanding greater rigor in moral language and reinforcing the distinction between facts and values.

Introduction: Setting the Stage in Meta-Ethics

Before we can understand what is right or wrong (Normative Ethics) or how to solve specific moral dilemmas (Applied Ethics), we must first ask a more fundamental question: What do we even mean when we use words like 'good', 'bad', 'right', and 'wrong'? This is the domain of meta-ethics, the branch of philosophy that studies the nature, scope, and meaning of moral language and judgments. At the dawn of the 20th century, British philosopher G.E. Moore revolutionized this field with his 1903 book, Principia Ethica. In it, he identified what he considered a pervasive and fundamental error in most previous ethical theories: the Naturalistic Fallacy.

What is the Naturalistic Fallacy?

The Naturalistic Fallacy is the error of attempting to define the concept of 'good' in terms of some 'natural' property or set of properties. Moore argued that 'good' is a unique and simple concept that cannot be broken down or equated with anything else.

What did Moore mean by a 'natural' property? A natural property is one that exists in the physical world and can be observed, measured, or studied by the empirical sciences (like physics, biology, or psychology). Examples of natural properties include:

• Pleasure or happiness (central to Utilitarianism)
• That which is desired or preferred (a subjective view)
• That which is more evolved or aids survival (central to Social Darwinism)
• That which fulfills a function or purpose (an Aristotelian view)

According to Moore, any philosopher who claims "'Good' means the same thing as 'pleasurable'" or "'Good' is defined as 'that which aids survival'" is committing the naturalistic fallacy. They are confusing the property of goodness itself with other properties that good things might happen to possess. For Moore, saying "pleasure is good" is not a definition; it is a substantive moral claim that requires justification. It asserts that two different properties, pleasure and goodness, are found in the same thing.

The Open-Question Argument: Moore's Litmus Test

To prove that 'good' cannot be defined by any natural property, Moore developed a powerful and intuitive line of reasoning known as the Open-Question Argument. The argument works as a test for any proposed definition of 'good'.

Let's take a naturalistic definition, such as the hedonistic claim: "'Good' means 'that which causes pleasure'."

1. If the definition is correct, the two terms ('good' and 'that which causes pleasure') are synonymous. They mean exactly the same thing.
2. Therefore, the question, "Is that which causes pleasure, good?" should be a closed question. A closed question is one where the answer is self-evident from the terms used. For example, asking "Are bachelors unmarried men?" is a closed, nonsensical question because 'bachelor' is defined as an 'unmarried man'. The answer is an obvious and uninformative "yes."
3. However, Moore observes that the question, "Is that which causes pleasure, good?" is always a significant and open question. We can meaningfully ask it. We can debate whether a sadist's pleasure is truly 'good', or whether a difficult but character-building experience (which is not pleasurable) might be 'good'. The fact that we can sensibly doubt whether a pleasurable thing is good shows that the two concepts are not identical.
4. Conclusion: Since the question remains open, the proposed definition must be false.

This argument can be applied to any natural property (X). For any proposed definition where 'good' means 'X', we can always meaningfully ask, "But is X really good?" The question never becomes a pointless tautology. Therefore, Moore concludes, 'good' cannot be defined by any natural property whatsoever.

Moore's Conclusion: Good as a Simple, Non-Natural Property

If 'good' is indefinable, then what is it? Moore's answer is that 'good' is a simple, non-natural property.

• Simple: Like a primary color, it cannot be broken down into constituent parts. Moore uses the famous analogy of the color 'yellow'. You cannot explain what 'yellow' is to someone who has never seen it. You can point to yellow objects—a lemon, a taxi, a sunflower—and say, "This is yellow." But you cannot define the color itself in terms of anything else. Similarly, we can't define 'good', but we can recognize it when we see it.
• Non-Natural: 'Goodness' is not a property that exists in time or space or can be detected by our five senses or scientific instruments. It does not belong to the world of natural facts. It is a unique, sui generis (of its own kind) property.

If we can't define or observe 'good', how do we know what is good? Moore's answer is that we apprehend it through a special faculty of moral intuition. We can directly intuit or recognize the presence of this non-natural property of 'goodness' in certain actions or states of affairs. This position is known as Ethical Intuitionism.

Criticisms and Legacy

Moore's argument, while hugely influential, is not without its critics. The most common objection is that the Open-Question Argument begs the question—it assumes the conclusion it's trying to prove. A committed naturalist could simply retort that the question "Is pleasure good?" is a closed question for them, and anyone who thinks it's open is simply confused about the meaning of 'good'.

Furthermore, the reliance on 'intuition' is problematic. People have wildly different moral intuitions. If two people have conflicting intuitions about whether something is 'good', Moore's theory offers no clear mechanism for resolving the disagreement, potentially leading to a stalemate of competing, unprovable claims.

Despite these criticisms, the impact of the naturalistic fallacy cannot be overstated. It single-handedly shifted the focus of 20th-century meta-ethics towards a rigorous analysis of moral language. It drew a sharp line between factual statements ('is') and value judgments ('ought'), reinforcing a distinction famously made by David Hume. By challenging the foundations of ethical naturalism, Moore's work paved the way for non-cognitivist theories (like emotivism and prescriptivism), which agreed that 'good' was indefinable but concluded that moral statements were not truth-claims about properties at all, but rather expressions of emotion or commands.

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