Moore's Defense of Common Sense and Critique of Idealism

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Introduction

Welcome to our final lesson in the module on the "Logical Turn." In our previous lessons, we've focused heavily on the formal, logic-driven philosophy of Frege and Russell. You've become proficient in applying Russell's Theory of Descriptions, a powerful tool for analyzing sentences by revealing their underlying logical structure.

Today, we turn to the third major figure in the founding of analytic philosophy, G.E. Moore. While a contemporary and colleague of Russell's at Cambridge, Moore's approach was strikingly different. Instead of building complex logical systems, he championed a philosophy grounded in common sense and the careful analysis of ordinary language.

Our goal for this lesson is to describe G.E. Moore's defense of common sense and his critique of idealism. We'll see how Moore first sought to dismantle the dominant idealist philosophy of his time, and then how he used this cleared ground to build a new philosophical method centered on truths he believed we all already know. This will offer a valuable contrast to Russell's formalism and complete our picture of early analytic philosophy's diverse origins.

1. The Critique of Idealism

At the turn of the 20th century, British universities were dominated by British Idealism, a school of thought influenced by German philosophers like Hegel. Idealists like F.H. Bradley and J.M.E. McTaggart held that reality is fundamentally mental or spiritual. Their metaphysical systems often led to conclusions that starkly contradicted the everyday view of the world—for instance, McTaggart famously argued that time is unreal.

A central tenet underlying many idealist arguments, famously articulated by George Berkeley, is the principle esse est percipi: "to be is to be perceived." The idea is that nothing can exist unless it is being experienced or thought of by a mind. Moore’s first major contribution to philosophy was a direct assault on this principle.

The "Refutation of Idealism" (1903)

In his seminal paper, Moore argues that idealists commit a fundamental error: they fail to properly distinguish the act of awareness from the object of awareness.

Let's take the example of a "sensation of blue." Moore contends that this experience involves two distinct components:

Consciousness: The act of being aware, which is common to all sensations (whether of blue, green, hot, or cold).
The Object: The specific thing you are aware of, in this case, 'blue'.

The idealist, according to Moore, conflates these two. They treat 'blue' as a mere content of the sensation, an inseparable part of the mental event itself. If 'blue' is just a property of the sensation, then it can't exist apart from the sensation.

Moore's crucial insight is that this is a mistake. He describes consciousness as "diaphanous" or transparent. When you try to introspect your sensation of blue, you don't "see" your awareness; you see blue. Your awareness is directed at blue.

Once this distinction is made, the idealist argument collapses. The object ('blue') is distinct from the act of being aware of it. If they are distinct, there is no longer any reason to think that the existence of the object depends on the act of awareness. As Moore famously concludes, "Merely to have a sensation is already to be outside the circle of our own ideas and sensations."

To get a more detailed account of Moore's arguments against the various forms of idealism, the following reading is highly recommended.

George Edward Moore - Stanford Encyclopedia of Philosophy

To understand the intellectual climate Moore was reacting against and the details of his counter-argument, please read the following section from the Stanford Encyclopedia of Philosophy.

Please read section 2, 'The Refutation of Idealism'. Focus on how Moore distinguishes between the mind and its objects, and how he applies this distinction to criticize the idealist position, particularly in his analysis of a 'sensation of blue'.

2. The Defense of Common Sense

Having attacked the foundations of idealism, Moore established a new starting point for philosophy: the certainty of our ordinary, common-sense beliefs.

"A Defence of Common Sense" (1925)

In this paper, Moore lists a series of propositions that he calls "truisms," which together constitute the "Common Sense view of the world."

Summary of G.E. Moore's A Defence of Common Sense — This image provides a clear summary of Moore's Common Sense Propositions (CSPs). These are statements like "There exists at present a living human body which is my body" and "The earth had existed also for many years before my body was born."

Moore makes two key claims about these truisms:

He knows them to be true with certainty.
Many other people also know their corresponding truisms to be true with certainty.

His radical move is to insist that our certainty in these common-sense beliefs is far greater than our certainty in any philosophical premise that could be used to call them into doubt. If a philosophical theory (be it idealism or skepticism) concludes that you don't have a body or that the external world doesn't exist, then that theory must be wrong—not common sense.

"Proof of an External World" (1939)

Moore's most famous, and perhaps most provocative, demonstration of his philosophy came in a lecture where he offered a "proof" of an external world to counter skeptics. The proof is strikingly simple:

(While holding up one hand) "Here is one hand."
(While holding up the other) "And here is another."
Therefore, two external objects exist, and an external world exists.

To many, this seems absurdly simplistic or to be begging the question. But Moore argued it was a "perfectly rigorous proof" because it met three conditions:

The premises are different from the conclusion.
The conclusion logically follows from the premises.
The premises are things he knows to be true.

The controversy, of course, hinges on the third condition. How can he claim to know the premises in the face of skeptical arguments (e.g., "How do you know you're not a brain in a vat being fed the sensation of hands?").

Given your background in logic, Moore's strategy can be elegantly framed as a reversal of the skeptic's argument.

The Skeptic's Argument (as modus ponens)
- Premise 1: If I don't know that I'm not a brain in a vat (BIV), then I don't know that I have hands.
- Premise 2: I don't know that I'm not a BIV.
- Conclusion: Therefore, I don't know that I have hands.
Moore's Reversal (as modus tollens)
- Premise 1: If I don't know that I'm not a BIV, then I don't know that I have hands. (Moore agrees with this conditional.)
- Premise 2: But I do know that I have hands.
- Conclusion: Therefore, I do know that I'm not a BIV.

Both arguments are valid in form. The disagreement lies in which premise (the skeptic's P2 or Moore's P2) is more plausible. Moore's central claim is that his common-sense proposition, "I know I have hands," is vastly more certain than the abstract philosophical claim, "I don't know that I'm not a brain in a vat." He essentially shifts the burden of proof to the skeptic.

The following video provides an excellent, in-depth discussion of this proof, its context, and its philosophical significance.

G.E. Moore: "Proof of an External World"

The following video explains the context and structure of Moore's famous 'Proof of an External World', framing it as a direct response to skepticism.

Watch from 03:13 to 23:17. Pay close attention to the explanation of the skeptical challenge (the 'brain in a vat') and how Moore flips the skeptic's argument using 'modus tollens'. Also, note the discussion of 'Moorean facts'—things you can know without being able to prove.

3. The Proper Task of Philosophy

So, if the job of philosophy isn't to prove or disprove the existence of the external world, what is it? For Moore, the answer is analysis.

While we can be certain that common-sense propositions are true, we are often not clear on what their correct analysis is. When Moore says, "I know that here is a hand," he is not claiming to have a perfect philosophical understanding of the proposition. The job of the philosopher is to analyze the concepts involved: What is a 'hand'? What is a 'material object'? What is the relationship between my sense-data of the hand and the hand itself?

This is Moore's connection to the broader project of analytic philosophy. He shifts the goal from proving grand metaphysical theories to achieving clarity on the meaning of propositions we already accept as true.

This short video neatly summarizes this metaphilosophical point.

G.E. Moore - Common Sense and the "Proof" of the External World

This short video provides a great summary of the metaphilosophical point behind Moore's proof—what he thought the purpose of philosophy should be.

Watch from 00:58 to 04:42. Focus on the idea that philosophy's job isn't to overturn common sense, but to analyze and explain how we know the things that common sense tells us.

Test your understanding!

Imagine Bertrand Russell and G.E. Moore are both asked to respond to the statement: "The golden mountain does not exist."

Based on what you've learned about their respective methods, how would their approaches differ? What would each philosopher focus on?

Show answer

This question highlights the core difference in their philosophical methods.

Russell's Approach (Logical Analysis):
Russell would treat "The golden mountain" as a definite description. His primary goal would be to analyze the sentence's logical form to show how it can be meaningful and true without committing us to the existence of a strange entity (a non-existent golden mountain). Using his Theory of Descriptions, he would translate the sentence as the negation of an existence claim:

Logical Form: ¬∃x (G(x) ∧ M(x) ∧ ∀y((G(y) ∧ M(y)) → y=x))
Translation: "It is not the case that there is one and only one thing that is both golden and a mountain."

For Russell, the analysis is successful because it reveals the sentence's true logical structure, solves the puzzle of denoting non-existent objects, and demonstrates the power of his logical method. The focus is on formal translation and logical hygiene.

Moore's Approach (Common-Sense Analysis):
Moore would start from the opposite end. He would first affirm that "The golden mountain does not exist" is a proposition he knows to be true as a matter of common sense. He would not feel the need to prove this via a complex logical formula.

His philosophical task would then be to analyze what we mean by such a statement. He might ask questions like:

What is the analysis of 'exist'?
What is the difference in our understanding when we say "the golden mountain does not exist" versus "lions exist"?
When we think of 'the golden mountain', what are we thinking of? Is it an 'idea' or a 'concept'?

Moore's focus is on clarifying the meaning of the common-sense belief itself, not on translating it into a formal language to prove its logical coherence. He assumes its truth and proceeds to analyze its meaning.

Conclusion

Today we've explored the philosophy of G.E. Moore, a crucial counterpoint to the formalist wing of early analytic philosophy. We've seen how his work provided a powerful defense of realism and common sense against the prevailing idealism and skepticism of his era.

Key Takeaways:

Critique of Idealism: Moore argued that idealists mistakenly conflate the act of awareness with the object of awareness. By distinguishing consciousness from its object, he dismantled the core idealist principle of esse est percipi.
Defense of Common Sense: Moore championed the view that certain common-sense beliefs are more certain than any philosophical argument that could be used to deny them.
The "Proof" as a Reversal: His famous "proof of an external world" is best understood not as a naive proof, but as a sophisticated reversal of the skeptic's argument, shifting the burden of proof by privileging common-sense certainty.
Philosophy as Analysis: For Moore, the proper task of philosophy is not to prove or disprove the common-sense view of the world, but to provide a clear analysis of the propositions we already hold to be true.

Preview of the next lesson:

Having examined the foundational work of Frege, Russell, and Moore, we will next explore a movement that drew heavily from their influence: Logical Positivism. In the first lesson of the next module, we will explore the early philosophy of Ludwig Wittgenstein and his 'picture theory' of meaning, which profoundly shaped the Logical Positivists' radical, anti-metaphysical project.