 Constructive empiricists recognize that these pragmatic factors like simplicity and explanatory power are important guides in the pursuit of the aim of science (van Fraassen 1980, 89). But they insist that these factors are valuable in that pursuit only insofar as their consideration advances the development of theories that are empirically adequate and empirically strong. The factors do not have special value as indicators of the truth of what the theories say about the unobservable parts of the world.