This article records tradition as it has been passed down and reported. Its sources are not yet part of the atlas's verified catalogue.
G.E. Moore's 1903 essay The Refutation of Idealism set out to dismantle the central premise of the philosophy that had dominated British universities for a generation, the idealism associated above all with F.H. Bradley, according to which reality is at bottom spiritual or mental and physical objects have no existence apart from being experienced. Moore isolated the argument's hidden engine in a single Latin tag the idealists relied on, esse est percipi, to be is to be perceived, and set out to show that the inference from to be is to be experienced to reality is mental trades on an ambiguity that, once exposed, leaves idealism without support.
Moore's method was close attention to a single case, the sensation of blue. Ask what happens when someone sees a blue patch and introspects the experience itself, rather than the blue object. The idealist wants to say that in such a case the sensation and its object, the blue, are one and the same thing, so that the blue turns out to be as mental as the seeing of it. Moore denied this. When we try to inspect the experience of blue in itself, apart from the blue that is experienced, the introspective act finds nothing there to inspect. In trying to fix attention on consciousness itself, it seems to vanish, it seems as if we had before us a mere emptiness, and when we try to introspect the sensation of blue, all we can see is the blue. This is the famous claim that consciousness is diaphanous, transparent to its object, so that whatever we find when we look for the experience turns out to be nothing but the blue itself, never some further mental item called the sensation over and above it.
From this observation Moore drew a logical point against identifying the object with the experience. If blue really were a mere quality of the sensation, in the way that a bead can be said to be blue, then talk of the sensation of blue would amount to something like talk of the blue sensation, treating blueness as a property the mental state itself possesses. Moore called this comparison absurd, since a sensation and a color belong to entirely different categories, and insisted instead that the blue is an object the sensation is of, standing over against the experience rather than constituting it. Once the object is prised apart from the act of experiencing it in this way, the idealist's premise collapses, since the blue is shown to be exactly as independent of being perceived as any object that idealism wanted to declare impossible, an item as much an object, and as little a mere content, of experience, as the most exalted and independent real thing anyone is ever aware of.
Moore recognized later that the argument, stated so starkly, ran into difficulty with cases of illusion and hallucination, where the introspected quality apparently has no independently existing thing answering to it at all, and he spent much of his later career refining rather than repudiating the position. But the essay's lasting effect on early analytic philosophy was to make direct realism, the view that perception puts us in unmediated contact with a mind independent world, a live option again after decades in which idealism had made it seem naive. It gave Moore and Russell, working closely together at Cambridge in this period, a shared reason to treat ordinary physical objects, universals, and other minds as fully real constituents of the world rather than constructions out of experience, a conviction that shaped the whole subsequent direction of British philosophy. Moore's insistence that the deliverances of common sense, that tables, chairs, and other people exist independently of anyone perceiving them, deserved more credence than any clever philosophical argument purporting to prove otherwise, found its first sustained defense in this essay, years before the more famous proof of an external world he offered later in his career, and it remains the essay in which Moore's characteristic method, patient attention to a single ordinary case rather than a grand system, first showed what it could do against an entrenched philosophical orthodoxy.