If my interests, practical or speculative, are not affected by a certain trial now proceeding in the law courts, I can avoid making any assumption as to the trustworthiness or untrustworthiness of a witness's evidence. I do not know whether he is trustworthy or not, and I can refuse to make any assumption whatever on the subject--there is no reason why I should. But the moment circumstances call on me to form an opinion, I find myself beginning to make one assumption or the other, or perhaps at first one and then the other, though I am just as ignorant whether he really is trustworthy or not as I was when I refused to make any assumptions; I know no more about his previous career or his antecedent credibility than I did before he entered the box. So, too, the truth is not that science makes no assumptions, but that she makes no assumptions except those which are necessary for her purposes. The man of science assumes--and it is pure assumption--that he can trust the evidence of his consciousness as to the reality of the chemicals he experiments on, the plants he classifies, or the stars he observes. He assumes that they are real. He also assumes without proof that what has produced a certain effect once will produce it again in the same circumstances, that if a thing has occurred the conditions essential to its occurrence must also have occurred--in fine, that Nature is uniform. But so long as he is engaged exclusively in scientific work, in finding out what actually does happen or has happened in Nature, he need make no assumptions as to whether a certain witness is trustworthy or not, or whether there is a God or not: he can maintain a perfectly agnostic attitude on both questions. He can say, if he chooses, "God or no God, two and two make four"; or, to put it more precisely, whether the evidence which consciousness gives in spiritual experience to the reality of a God can or cannot be trusted, I do trust the evidence which consciousness gives in sense-experience to the reality of material things; whether the assumption that every event is the expression of self-determining will is true or not, at any rate I believe in the assumption that Nature is uniform.