In Britain, idealism ruled the day, which is basically just a philosophical way of saying that the world was more about the ideal truth than it was about the real observable world. In Proof of an External World, Moore disagrees with his colleagues and mentors by asserting that there is a common sense argument for the existence of the external world (meaning that we're in the question of existential solipsism).
Solipsism is the unfortunate belief that the person in question is the only "real" person, and that nothing external to themselves can be known with any real certainty, because after all, Descartes was fairly adamant on this point. Moore argues that skepticism and idealism were counterintuitive because they depend on opinion alone, being that no one can provide definitive arguments about such speculative issues.
He continues by dramatizing his opinion with a metaphor. He famously lifts his hands and says there must be at least two external objects in the world, because his knowledge of his hand corresponds to his experience of his hand as part of external reality. That is to say he knows that there is a real world because he himself is an object in that world observably.
Although it didn't change many people's minds about the issue, it is certainly an important contribution and most importantly of all, Moore's Proof seeks to reconnect speculative philosophy with real life experience, rooting it in common sense and a commitment to reality.
