An experiment held to decide with certainty between two rival hypotheses about some matter. Famous examples include Eddington’s observation of the bending of light rays by the sun during the eclipse of 1919, held to decisively uphold general relativity against Newtonian mechanics, or the observation of weight gain during combustion, held to decide for the theory that combustion is oxidation and against the view that it consists in loss of phlogiston. In practice experiments require a great deal of scene-setting and agreement on what would count as an ad hoc hypothesis before they play such a decisive role, so that rational disagreement on whether one or another rival is really refuted is both possible in theory and often found in practice.