It is quite common, in reading and in conversation, to find references to the “scientific mind,” but it is difficult to ascertain precisely how this mental structure is supposed to differ from other sorts of mind. The difficulty of defining an object does not, perhaps, affect the probability of the existence of the object; although it is difficult for some people to refrain from concluding that because a man cannot define what he means he does not mean anything. We must suppose that there is some particular kind of mind called the scientific mind, in spite of the fact that the numerous references to it tell us little about it except that it is somewhat extensively disliked. So far as can be judged from a superficial comparison of different references, the “scientific mind” is characterised by an inordinate appetite for facts and an absence of generosity in drawing conclusions from facts. In ordinary times this absence of generosity is dismissed by most people as quibbling, while in time of war it becomes unpatriotic. During the war every Englishman was supposed to believe a great number of things on very slender evidence or even on no evidence. It was considered that a right patriotic feeling not only could, but should, supply the place of evidence, and lead to correct conclusions. The majority of people in every class of the community found themselves able to adopt this method of thought without discomfort, and it became evident that the scientific mind is as rare amongst scientific men as amongst any other men, while those who could not give this supreme proof of patriotism were found pretty evenly distributed amongst the different classes. As a type of mind, therefore, it is not peculiar to scientific men nor do they all possess it. It cannot be regarded as a distinguishing mark of this class. But while a just, cautious temperament need not belong to the man of science as a human being, it might be thought that, as a mental habit, it is necessary to his work. There is much truth in this, although it is not wholly true. Alternative explanations are not always explored by scientists, and if, as sometimes happens, the alternative explanations are wrong, the scientific man may have reached a correct result although he worked in a partisan spirit.