These problems even the beginner may be willing to recognize as philosophical; but he may conscientiously harbor a doubt as to the desirability of spending time upon the solutions which are offered. System rises after system, and confronts him with what appear to be new questions and new answers. It seems as though each philosopher were constructing a world for himself independently, and commanding him to accept it, without first convincing him of his right to assume this tone of authority and to set up for an oracle. In all this conflict of opinions where shall we seek for truth? Why should we accept one man as a teacher rather than another? Is not the lesson to be gathered from the whole procession of systems best summed up in the dictum of Protagoras: "Man is the measure of all things"--each has his own truth, and this need not be truth to another? This, I say, is a first impression and a natural one. I hasten to add: this should not be the last impression of those who read with thoughtful attention. One thing should be emphasized at the outset: nothing will so often bear rereading as the history of philosophy. When we go over the ground after we have obtained a first acquaintance with the teachings of the different philosophers, we begin to realize that what we have in our hands is, in a sense, a connected whole. We see that if Plato and Aristotle had not lived, we could not have had the philosophy which passed current in the Middle Ages and furnished a foundation for the teachings of the Church. We realize that without this latter we could not have had Descartes, and without Descartes we could not have had Locke and Berkeley and Hume. And had not these lived, we should not have had Kant and his successors. Other philosophies we should undoubtedly have had, for the busy mind of man must produce something. But whatever glimpses at the truth these men have vouchsafed us have been guaranteed by the order of development in which they have stood. They could not independently have written the books that have come down to us. This should be evident from what has been said earlier in this chapter and elsewhere in this book. Let us bear in mind that a philosopher draws his material from two sources. First of all, he has the experience of the mind and the world which is the common property of us all. But it is, as we have seen, by no means easy to use this material. It is vastly difficult to reflect. It is fatally easy to misconceive what presents itself in our experience. With the most earnest effort to describe what lies before us, we give a false description, and we mislead ourselves and others.