 During the Second World War, separated from his Oxford philosophical brethren, and exposed to political action, Berlin began to drift away from his early philosophical concerns. His doubts were encouraged by a meeting with the Harvard logician H. M. Sheffer, who asserted that genuine increase in knowledge was possible only in such hybrid subfields of philosophy as logic and psychology. His meeting with Sheffer led Berlin to realise that he lacked the passion and the belief in his own ability to continue pursuing pure philosophy. He concluded that as a professional philosopher (as he then understood that role) he would make no original contributions, and would end his life knowing no more than he did when he began. He therefore determined to switch to the history of ideas, in which (he believed) originality was less essential, and which would allow him to learn more than he already knew. Berlin’s approach to the history of ideas would, however, remain deeply informed by his philosophical persona, as well as by his political beliefs. His historical work was, in effect, the practice of philosophy in a historical key.