 Thus far, the argument for theoretical pluralism largely follows that of John Stuart Mill’s On Liberty (1859), to which Feyerabend often paid homage. But Feyerabend went on to try to demonstrate a mechanism whereby theories can augment their empirical content. According to this part of the argument, theories may face difficulties which can only be discovered with the help of alternative theories. A theory can be incorrect without our being able to discover this in a direct way: sometimes the construction of new experimental methods and instruments which would reveal the incorrectness is excluded by laws of nature, sometimes the discrepancy (were it to be discovered) might be regarded as an oddity, and might never be given its correct interpretation. Circumstances can thus conspire to hide from us the infirmities of our theory. The methodological “principle of testability” demands that we develop alternative theories incompatible with the existing theory, and develop them in their strongest form, as descriptions of reality, not mere instruments of prediction. Instead of waiting until the current theory gets into difficulties, and only then starting to look for alternatives, we ought vigorously to proliferate theories and tenaciously defend them in the hope that they may afford us an indirect refutation of our existing theory. Only theories which are empirically adequate will thus contribute to raising the empirical content of their fellows. But Feyerabend insists that any theory, no matter how weak, may become empirically adequate, and so may contribute to this process. To be a realist, he therefore suggests, involves demanding support for any theory, including implausible conjectures having no independent empirical support, conjectures which are inconsistent with data and well-confirmed laws. We should retain theories that are in trouble, and invent and develop theories that contradict the observed phenomena, just because in doing so we will be respecting the intellectual ideal of testability.