little until he has learned to weigh its general credibility by his own standards, and its particular credibility by subjecting portions of it to his own tests; to distrust it still more when even small portions fail to answer his tests, and to reject it altogether when the percentage of detected error is large. He learns, in fact, what Huxley called the duty of doubt. This duty has not been universally accepted. In the history of Christian civilisation (and a parallel series of events might be portrayed from the history of other civilisations), many great institutions and very many great and good men have condemned and feared the habit and attitude of doubt in all its forms. Certain doctrines believed to be of supreme importance to mankind were held to rest on authority independent of, and perhaps not susceptible to, the kind of testing employed in science. Around these doctrines there grew, in time, a body of traditions, customs, new dogmas, and fantasies; and the duty of belief in the first was extended to cover the whole system, the central jewel as well as the accretions and encrustations of time. The domain of religious authority was extended to the whole field of human thought and of human action, and the more unreasonable the dominion became, the more strenuously was the duty of belief urged. The Protestant Reformation was one of the great stages in the conflict for freedom against the universal tyranny that had arisen, but the reformers very naturally retained a considerable portion of the bias against which they had fought. In Protestant countries, in the first half of this century, the duty of belief in the Protestant doctrines, traditions, philosophy, history, and attitude to science reigned supreme, and all weapons, from legitimate argument to abusive invective and social ostracism, were employed against those who acted in accordance with the duty of doubt. Allegations of "unsoundness" or of "free thinking" became barriers to success in life, and those against whom they were made became lowered in the esteem of their fellows. At the present time, when the advance of science and of civilisation has almost won the battle for freedom of thought, it is difficult to realise the strength of the forces against which Huxley and many others had to fight. Huxley himself said with perfect justice: "I hardly know of a great physical truth whose universal reception has not been preceded by an epoch in which most estimable persons have maintained that the phenomena investigated were directly dependent on the Divine Will, and that the attempt to investigate them was not only futile but blasphemous." As a particular instance of this he cited some episodes in the history of geological science.