Historians are able to select and create evidence by using some theory of human motivations and behavior. Sometimes this appears to be easy, requiring very little sophistication and subtlety. Thus, for example, historians investigating America's entry into World War I would probably find that the sinking of American merchant ships on the high seas by German submarines was relevant to their discussion. At the same time, they would most likely not use evidence that President Woodrow Wilson was dissatisfied with a new hat he bought during the first months of 1917. The choice as to which fact to use is based on a theory—admittedly, in this case a rather crude theory, but a theory nonetheless. It would go something like this: National leaders contemplating war are more likely to be influenced by belligerent acts against their countries than by their unhappiness with their haberdashers.

If the choices were as simple as this, the problem would be easily resolved. But the choices were not so easy to make. Historians investigating the United States' entry into World War I will find in addition to German submarine warfare a whole series of other facts that could be relevant to the event under study. For instance, they will find that the British government had a propaganda machine at work in the United States that did its best to win public support for the British cause.