 The rise of particle physics and quantum mechanics in the 20th Century led to various principles of plenitude being appealed to by scientists as an integral part of their theoretical framework. A particularly clear-cut example of such an appeal is the case of magnetic  monopoles.[24]  The 19th-century theory of electromagnetism postulated numerous analogies between electric charge and magnetic charge. One theoretical difference is that magnetic charges must always come in oppositely-charged pairs, called “dipoles” (as in the North and South poles of a bar magnet), whereas single electric charges, or “monopoles,” can exist in isolation. However, no actual magnetic monopole had ever been observed. Physicists began to wonder whether there was some theoretical reason why monopoles could not exist. It was initially thought that the newly developed theory of quantum mechanics ruled out the possibility of magnetic monopoles, and this is why none had ever been detected. However, in 1931 the physicist Paul Dirac showed that the existence of monopoles is consistent with quantum mechanics, although it is not required by it. Dirac went on to assert the existence of monopoles, arguing that their existence is not ruled out by theory and that “under these circumstances one would be surprised if Nature had made no use of it” (Dirac 1930, p. 71, note 5). This appeal to plenitude was widely—though not universally—accepted by other physicists.