**Theology Defined.** Since all men are placed in the same universe, with approximately the same powers, and under conditions nearly alike, it is possible for each person to establish for himself a religion as above defined, for the guidance of his life. All religions must be organized from the content of the one, and so far as we know, the only universe; and the presumption would be, therefore, that the religions of all men should be the same, in as far at least as men are the same. In fact, however, during the course of human history, many more or less dissimilar religions have been established and accepted. True, most of these religions show close kinship, but the vital differences are often very great. For instance, the religions of men fall naturally into two great classes: those that adopt as their central idea a great governing intelligence and power--a God; and those that refuse to include a God in their system of thought. A religion which accepts the idea of a God is a theology. The great majority of the religions of men are theologies, for the majority of men believe in some form of personal divine power. **The Gospel.** The word gospel is also frequently used, among Christians, to designate the religion of men. The Gospel is a theology which includes the doctrine of the life and mission of Jesus Christ, as the Son of God. Among Christians, the words religion, theology and gospel are freely used in the same sense. It is well, however, to bear in mind the distinction in meaning of the three words. The Christian religion, the Christian theology and the Gospel are equivalent in meaning. In the following pages, the terms are often used interchangeably without the qualifying words. Indeed, the Gospel will be used most frequently, and wherever used, it must be understood to stand for the rational theology discussed in this book. **The Purpose of This Book.** This volume is devoted to the exposition of the fundamental principles of a rational theology--a philosophy of life which, because of its complete harmony with all knowledge, should be the one to which all men might give adherence. CHAPTER 2. HOW KNOWLEDGE IS GAINED. Knowledge is the material upon which the reasoning mind of man acts. Just as physical strength can neither be developed nor exercised unless material bodies are at hand, so mental strength can neither be developed nor exercised unless facts or knowledge are in man's possession. The acquisition of knowledge or experience is the first step towards formulating an acceptable religion. It is of interest, therefore, to consider, briefly, the sources of human knowledge.