 Popper therefore argues that there are no statements in science which cannot be interrogated: basic statements, which are used to test the universal theories of science, must themselves be inter-subjectively testable and are therefore open to the possibility of refutation. He acknowledges that this seems to present a practical difficulty, in that it appears to suggest that testability must occur ad infinitum, which he acknowledges is an operational absurdity: sooner or later all testing must come to an end. Where testing ends, he argues, is in a convention-based decision to accept a basic statement or statements; it is at that point that convention and intersubjective human agreement play an indispensable role in science: