Popper argues that science should adopt a methodology based on falsifiability, because no number of experiments can ever prove a theory, but a reproducible experiment or observation can refute one. According to Popper: "non-reproducible single occurrences are of no significance to science. Thus a few stray basic statements contradicting a theory will hardly induce us to reject it as falsified. We shall take it as falsified only if we discover a reproducible effect which refutes the theory". Popper argues that science should adopt a methodology based on "an asymmetry between verifiability and falsifiability; an asymmetry which results from the logical form of universal statements. For these are never derivable from singular statements, but can be contradicted by singular statements"