Philosophers seem to me entirely right if they think that this is what a system ought to be; but not entirely right if they think that this is what any system is, or has ever been. In any case, no description could be less applicable to the point of view which I am provisionally recommending. The philosopher refuses—in theory—to assume anything which requires proof. I assume (among other things) the common-sense outlook upon life, and the whole body of the sciences. The philosopher admits—in theory—no ground of knowledge but reason. I recognise that, in fact, the whole human race, including the philosopher himself, lives by faith alone. The philosopher asks what creed reason requires him to accept. I ask on what terms the creed which is in fact accepted can most reasonably be held. The philosopher conceives that within the unchanging limits of his system an appropriate niche can be found for every new discovery as it arises. My view is that the contents of a system are always reacting on its fundamental principles, so that no philosophy can flatter itself that it will not be altered out of all recognition as knowledge grows. This last statement may look like a truism; but it is a truism which few philosophers are, in practice, disposed to accept; and the generality of mankind are perhaps even less disposed to accept it even than philosophers. That there are beliefs which can and should be held, with the same shade of meaning, by all men, in all ages, and at all stages of culture, is a view to which by nature we easily incline. But it is, to say the least, most doubtful. Language is here no true or certain guide. Even when beliefs have not outgrown the formulas by which they have been traditionally expressed, we must beware of treating this fixity of form as indicating complete identity of substance. Men do not necessarily believe exactly the same thing because they express their convictions in exactly the same phrases. And most fortunate it is, in the interests of individual liberty, social co-operation, and institutional continuity that this latitude should be secured to us, not by the policy of philosophers, statesmen, or divines, but by the inevitable limitations of language. This, however, by the way. The point I wish to press is that, speaking generally, we must not conceive the development of knowledge as a process of adding new truths to old truths, in the course of which old truths are supplemented but are not changed. It rather resembles the increase of some plastic body which, wherever it takes place, involves a readjustment of every part. Add brick to brick, and you may finish your house, yet never alter its foundation. Add belief to belief, and you will set up strains and stresses within your system of knowledge which will compel it to move towards some new position of equilibrium.