The idea of distinguishing between the internal and the external features of theories and regarding these features separately as potential indicators of the truth of a theory was only a start-up suggestion. It has served as a heuristic organizer of the activities of science and the steps of justification. Such organizing concepts are always needed for thinking about evidence, evidence in this case of the methods of scientific justification and of the structure of scientific knowledge. It seemed natural to begin by dividing the fund of information that could be used to evaluate a theory into two categories, depending on the source of the information. There could be information from theories themselves, both the theory being evaluated and other theories relevant to it, and information from the world. Thus justification of a theory would be a combination of theory-to-theory relations, as in the standards of entrenchment and explanatory cooperation, and theory-to-world relations. This matched the internal–external distinction.

The initial suggestion that this is a sharp dichotomy, separating distinctive kinds of features, different sources of information, and characteristic manners of justification, has become untenable in light of the closer look at the relationship between theories and evidence in science. The developing model of science is proposed and tested just as a theory in science. Just as in science, an initial hypothesis was a helpful background against which to structure the evidence, but the hypothesis is answerable to the evidence and it must be revised or rejected if the evidence demands.