The three contributions that Pepper makes in World Hypotheses are: first, the theory of development of knowledge (through the refinement of commonsense knowledge); second, the root metaphor theory (which explains the origin of the world hypotheses); third, the analysis of the six world hypotheses that drive all philosophical thought.
I find the root metaphor theory with which Pepper illustrates the origin of hypotheses of unlimited scope quite interesting. Here’s an excerpt which explains the nature of root metaphor theory and its relationship to the hypotheses of the world:
A man desiring to understand the world looks about for a clue to its comprehension. He pitches upon some area of commonsense fact and tries to understand other areas in terms of this one. This original area becomes his basic analogy or root metaphor. He describes as best he can the characteristics of this area, or, if you will, discriminates its structure. A list of its structural characteristics becomes his basic concepts of explanation and description. We call them a set of categories. In terms of these categories he proceeds to study all other areas of fact whether uncriticized or previously criticized. He undertakes to interpret all facts in terms of these categories . As a result of the impact of these other facts upon his categories, he may qualify or readjust the categories, so that a set of categories commonly changes and develops. Since the basic analogy or root metaphor normally (and probably at least in part necessarily) arises out of common sense, a great deal of development and refinement of a set of categories is required if they are to prove adequate for a hypothesis of unlimited scope. Some root metaphors prove more fertile than others, have greater powers of expansion and of adjustment. These survive in comparison with the others and generate the relatively adequate world theories. (Page: 91-92)