George Berkeley is best known for his denial of matter and for a series of arguments which, according to David Hume, "admit of no answer and produce no conviction." James Boswell offered a similar assessment: "though we are satisfied [Berkeley's] doctrine is not true," he said to Samuel Johnson, "it is impossible to refute it." He reported that Johnson disagreed: "I never shall forget the alacrity with which Johnson answered, striking his foot with mighty force against a large stone, till he rebounded from it,--'I refute it thus.' " Boswell interpreted Johnson's gesture as a "stout exemplification" of the first truths or original principles without which, Boswell insisted, "we can no more argue in metaphysicks, than we can argue in mathematicks without axioms." But any axiom can safely be expressed in words, and in choosing to act rather than argue, Johnson may have been suggesting that some...