As Popper describes it, World 3 is a product of language, particularly the descriptive and argumentative functions of language. Instruments are not linguistically built. Just like theories, organs and their functions are tentative adaptations to the world we live in. A direct reader surely is an exosomatic tool. Both theories and instruments express knowledge of the universe.
Theories express knowledge through the descriptive and argumentative functions of language.
Clearly both natural selection and the process behind the development of new theories and instruments are more complicated than this simple schema implies. Kuhn, Lakatos, and Feyerabend provide classic documentation of this point. The fit between product and the niche into which it is adapted, constitutes an expression of the knowledge this product provides of its niche. In either case, the resulting product exhibits knowledge of its particular niche, be it spectrochemical analysis of magnesium alloy production or capturing insects. There are crucial differences between the linguistic products with which Popper populates World 3 and the instruments with which I am concerned.