 Lewis dismisses alternatives to the pictorial conception, such as the “linguistic conception” (in which structural universals are set-theoretic constructions out of simple universals) and the “magic conception” (in which structural universals are not complex and are primitively connected to the relevant properties). Moreover, he also rejects the possibility of there being amphibians, i.e., particularized universals lying between particulars and full-fledged universals. Amphibians would solve the multiple recurrence problem, as they would be identical with multiple recurrences of single universals. For example, in methane, there would be four amphibians of hydrogen.