  The need to distinguish tokens of types from occurrences of types arises, not just in linguistics, but whenever types of things have other types of things occurring in them. There are 10,000 (or so) notes in Beethoven's Sonate Pathétique, but there are only 88 notes (types) the piano can produce. There are supposed to be fifty stars (types) in the current Old Glory (type), but the five-pointed star (type) is unique. And what could it mean to say that the very same atom (type), hydrogen, “occurs four times” in the methane molecule? Again, the perplexing thing is how the very same thing can “occur” more than once, without there being more than one token of it. Armstrong (1986), Lewis (1986a,b) and Forrest (1986) called such types “structural universals”, which were the subject of a debate among the three. Armstrong and Forrest defended Armstrong's (1978) view of universals against Lewis, who delineated seven different views of structural universals compatible with Armstrong's theory, and found all of them wanting. Basically, Lewis (1986a) assumed that there are two sorts of ways structural universals might be constructed of simpler universals: set-theoretically and mereologically. He argued that set-theoretical constructions resulted in ersatz universals, not universals worthy of the name, and that the various mereological constructions just resulted in a heap of simpler universals, where there could not be two of the simpler universals. Wetzel (2008) argues that there is a conception of a structural universal, the “occurrence conception”—which is an extension of the occurrence conception of expressions mentioned above—that escapes Lewis's objections.