G.E. Moore, answered from the texts and cited to the page.
The idealist formula "esse is percipi" — to be is to be perceived — confuses the act of perception with its object. When I perceive blue, there are two distinct things: my act of perceiving (which is mental) and the blue color that is its object (which is not mental, even if it is the object of a mental act). The idealist treats these as identical, or treats the object as if it were a constituent of the act.
The argument distinguishes them: the perceiving exists, the blue exists, and the relation between them is consciousness, not identity.1 Once the distinction is drawn, the idealist claim that the object cannot exist independently of being perceived loses its grip — the object is not part of the act and so its existence does not depend on the act's existence.2
The argument breaks the idealist picture of mind-dependent reality and establishes the realist alternative: there is a world of objects (colors, shapes, physical things, abstract entities) that exists independently of any mind that perceives or thinks of it. The position is one of the founding moves of analytic philosophy.345
The only alternative left, then, is that either both exist or the consciousness exists alone. If, therefore, any one tells us that the existence of blue is the same thing as the existence of the sensation of blue he makes a mistake and a self-contradictory mistake, for he asserts either that blue is the same thing as blue together with consciousness, or that it is the same thing as consciousness alone.Philosophical Studies, pp. 18–19
And this result I may express by saying that what is called the content of a sensation is in very truth what I originally called it - the sensation's object. But, if all this be true, what follows? Idealists admit that some things really exist of which they are not aware: there are some things, they hold, which are not inseparable aspects of their experience, even if they be inseparable aspects of some experience.Chapter 3: The Refutation of Idealism, p. 42
That they are believed, and that without this belief' esse is percipi' would not be believed either, I must leave without a proof. I pass, then, from the uninteresting question 'Is esse percipi? to the still more uninteresting and apparently irrelevant question 'What is a sensation or idea? We all know that the sensation of blue differs from that of green.Chapter 3: The Refutation of Idealism, p. 35
If, therefore, anyone tells us that the existence of blue is the same thing as the existence of the sensation of blue he makes a mistake and a self-contradictory mistake, for he asserts either that blue is the same thing as blue together with consciousness, or that it is the same thing as consciousness alone.Chapter 3: The Refutation of Idealism, p. 36
I shall only try to show that certain propositions which I assert to be believed, are false. That they are believed, and that without this belief 1 esse is percipi' would not be believed either, I must leave without a proof. I pass, then, from the uninteresting question ' Is 4 esse percipi? ' to the still more uninteresting and apparently ' irrelevant question ' What is a sensation or idea?Philosophical Studies, pp. 17–18
The 1903 Refutation is one of the foundational texts of analytic philosophy. The argument was decisive in its time against the British idealism of Bradley, Bosanquet, and McTaggart; combined with Russell's parallel anti-idealism in The Principles of Mathematics (1903), it cleared the ground for the realist analytic tradition that dominated twentieth- century Anglophone philosophy. Contemporary neo-idealists (Sprigge's The Vindication of Absolute Idealism 1983, Foster's The Case for Idealism 1982) have revived the idealist position; the disagreement is contemporary, not just a historical artifact. Baldwin's G.E. Moore (1990) Chapters I-II give the systematic contemporary exposition of the argument and its difficulties.