 Lewis (1986a) raises two problems for the pictorial conception. First, it is not clear how one and the same universal (e.g., hydrogen) can recur more than once in a structural universal. Let us dub this “multiple recurrence problem”. Secondly, structural universals violate the Principle of Uniqueness of Composition of classical  mereology.  According to this principle, given a certain collection of parts, there is only one whole that they compose. Consider isomers, namely, molecules that have the same number and types of atoms but different structures, e.g., butane and isobutane (C4H10). Here, butane and isobutane are different structural universals. Yet, they arise from the same universals recurring the same number of times. Let us dub this “isomer problem.”