Of all the sciences, philosophy is the most concrete and comprehensive. The sense of cold, remote spaces which it is apt to generate in us is the result of this very width and concreteness. The philosopher has to condense the many-sided variety of human life and express it through the symbols of a common language. The symbols are at best only semi-transparent. Abstractness descends upon him the moment they become opaque. Philosophy, in fact, is useless to us unless we can see through it. If we cannot, the fault may be ours. If it is the philosopher's, then his failure is a failure to be wide enough or concrete enough. He needs above all to be sympathetically sensitive to the wide range of human interests, because it is this as a whole that he is bidden to systematize. He must give a single answer, and for that he must formulate a single question. Therefore his first problem is to discover the unity of problems.