Operating theories are more or less concealed in the structure of all historical writing. They are readily discerned when the theories are comprehensive and at variance with generally accepted ideas, as in the case of Thomism or Marxism. They are hard to detect when like Spencerianism they conform closely to the mores of American life. Writers may honestly deny in this latter case that they have any special theory; they are simply recording the “facts” from a “common-sense” viewpoint. But since theory must be either implicit or explicit, it is better for scholarly purposes that it should be explicit. Carefully formulated theory restricts unconscious bias, gives meaning to otherwise formless data, and is more likely to reveal unexpected relationships.