 Moore and Russell found two main arguments for idealism to be fallacious. The first concerns Berkeley’s idealistic principle that being consists in being perceived, the second the converse claim, attributed to Bradley, that thought entails being. Their criticism of the first as well as their rebuttal of the second argument stems from certain convictions they share as to the nature of knowledge. The assault on Berkeley is staged by Moore most extensively in “The Refutation of Idealism” (1903). Here he holds that if there is an argument to prove the idealistic claim that the universe is spiritual (1903: 433) then this reasoning must rely either at the beginning or at some point later in the argument on the premise esse est percipi: 