Also, when you are testing so widely and carefully, and making sure of your foothold before every step you take, you come to realize the vast possibilities of different points of view. The one thing that the true scientist dreads is fixity, positiveness. Nature is forever mobile and flexible, and those who would follow nature and study her and interpret her must welcome and imitate her flexibility. They must be at all times ready to recognize the different aspects and phases of the same fact or group of facts, and willing to accept different conclusions with the changing and shifting light in which the facts are viewed. As Professor Whitehead puts it, admirably: ‘Science is even more changeable than theology. No man of science could subscribe without qualification to Galileo’s beliefs, or to Newton’s beliefs, or to all his own scientific beliefs of ten years ago.’[133] As I read in an excellent article in a field of scientific research very different from Darwin’s, that of philology and the emendation of classical texts: ‘In emending these passages we should adopt Pasteur’s method of investigation—exhaust every combination which it is possible for the mind of man to conceive.’[134] This impartiality, this breadth of view, this readiness to consider, if not to accept, all conjectures and all theories, is comparatively easy, when one is indifferent. If the motive of one’s investigation is mere curiosity, and one has no doctrine to establish, no thesis to defend, open-mindedness is facile and natural. But the scientific man instinctively forms theories, and when once a theory is formed, there comes the human impulse to maintain it, and to consider only those arguments and even those facts that will bear one out. As we have seen, it is here almost more than anywhere that Darwin’s example is of abiding value. No man could be more attached to a theory than he was to his. Yet he was determined, so far as human nature is capable of it, to keep his mind open and not to let his preconceptions color his observation or his reasoning. Huxley’s statement of the matter is indisputably correct when he speaks of Darwin’s ‘sagacity, his untiring search after the knowledge of fact, his readiness always to give up a preconceived opinion to that which was demonstrably true.’[135] And Darwin, in another of his simple, striking phrases, condenses all that open-mindedness means, and the difficulty of it and the rarity (italics mine): ‘If you argue about the non-acceptance of Natural Selection, it seems to me a very striking fact that the Newtonian theory of gravitation, which seems to every one now so certain and plain, was rejected by a man so extraordinarily able as Leibnitz. _The truth will not penetrate a preoccupied mind._’[136]