Moore’s “A Defence of Common Sense”


Common Sense Propositions





Subjective (Self)


Claims about the existence of my human body, where it has existed (at or near the surface of the earth), and when it has existed (born at a certain time in the past and has existed continuously ever since).

Claims about the existence and kind of experiences I have had—perceptions of material objects and facts about them, expectations of the future, true and false beliefs, thoughts about imaginary objects, illusory experiences.

Objective (Others)

Claims about the existence of other objects (including other human bodies), where they have existed (at some distance from or in contact with my body), and when.

Claims about other minds and the experiences they have had—other creatures with human bodies have had the same kinds of experiences I have had.


I.                     Moore’s Theses

A.      Two main claims

1.        Common Sense Propositions (CSPs) are true.

2.        Many people have known that CSPs are true.

B.       Clarifications

1.        CSPs are not partially true, they are wholly true.

2.        CSPs are true under their ordinary everyday meaning.

C.       Opponents

1.        Some or all of CSPs are not true.

a.        Some: Since matter or space is not real, all the material claims are false.

b.       All:  Since time or selves are not real, all CSPs (including all the mental claims) are false.

2.        Subjective mental claims are known to be true, but even if some of the other claims are true they are not known to be so.

D.      Common Sense Realism

1.        CSPs are true.

2.        We know that CSPs are true.

3.        Matter, space, time, and selves are real.

4.        Metaphysics: Analyze the ordinary meanings of CSPs to find out what objects/properties must exist.

5.        Epistemology: Analyze what we ordinarily mean when we say that we know CSPs to find out what knowledge is.



II.                   Moore’s Defense of His Two Main Theses

A.      CSPs are true

1.        Those who claim that matter/space/time/selves are not real hold inconsistent beliefs.

2.        The fact that philosophers have produced arguments that matter/space/time/selves provides no reason whatsoever to hold those views.

a.        If there have been philosophers who have argued for such claims, then those claims are false.



b.       The arguments that the reality of matter/space/time/selves leads to contradictions can be undermined:


P1:  If the reality of matter/space/time/selves led to contradiction, then some or all CSPs would lead to contradiction.


P2:  CSPs are true.


P3:  No true statements lead to contradiction.


C1:  So no CSPs lead to contradiction.


C2:  So the reality of matter/space/time/selves does not lead to contradiction.



B.       CSPs are known to be true

1.        The claim, “Many people have believed CSPs but they do not count as knowledge” is self-contradictory according to Moore.

a.        This claim is not logically self-contradictory.  But asserting the skeptical proposition is self-defeating.  Asserting, and thereby claiming to know, the first conjunct of the skeptical proposition is undermined by asserting the second conjunct.

b.       But what about skeptical arguments put in the first person and the claim “I do not know any material or objective CSPs”?


2.        Moore appears to claim to know CSPs simply on the basis that they appear certain to him and that he has no good reason to think that they are not true.







III.                 Certainty

A.      Descartes

1.        Knowledge requires certainty.

2.        Certainty requires being able to rule out any possibility of being false.

3.        This requires that a belief be infallible.

4.        S’s belief that P is infallible =df Not possible that S believes P and yet P is not true.


B.       Moore

1.        Knowledge requires certainty

2.        Certainty does not require being able to rule out any possibility that P is false.

3.        Certainty requires: Incorrigibility, Indubitability, Psychological Self-Evidence??

a.        S’s belief that P is incorrigible =df Not possible that someone can show S that P is false.

b.       S’s belief that P is indubitable =df Not possible for there to be rational grounds for S to doubt that P is true.

c.        S’s belief that P is psychologically self-evident =df P seems certain to S.







IV.                 Moore on the Problem of Skepticism

A.      Skeptical Arguments: The Moore Shift

1.        If the standard skeptical argument is P therefore C, where C denies that we know some CSP, then Moore will turn the argument on its head and make Not-C therefore Not-P.

2.        In other words, Moore will always give more credence to CSPs (and thus not-C) than to the premises of some skeptical argument (P).  Any argument that purports to show that we do not know some CSP must have something wrong with it.

3.        The Dream Argument:

a.        Moore agrees that if I really know there is a hand in front of my face, then I know I am not dreaming.

b.       Moore would argue however that he does know that he is not dreaming—because he knows that there is a hand in front of his face.


B.       Skeptical Aporia:

1.        Moore definitely accepts that knowledge requires certainty.

2.        Moore seems to accept the possibility that appearances can be massively in error.

3.        Moore definitely accepts that we know a lot about reality.

4.        Moore definitely says that we do not directly perceive objects themselves and that what we do directly perceive is a ‘sense-datum’, but he leaves open whether that sense-datum is identical with the surface of an object and the exact nature of the relationship between the sense-datum and the object it represents.