 The more significant complication, however, is that ontological relations need not be synchronic. For a mental state to be a memory, for instance, it must be caused by the event of which it is a memory. Likewise, a mark’s being a footprint partly depends on historical events: it requires that the mark was made by the strike of a foot (Dretske 1988, Stalnaker 1989). And for a person to be President of the United States, an election must have taken place beforehand. Some theories of the social world insist that a social entity can only ontologically depend on synchronic facts about the world. Classical structuralism, influenced by Saussure 1916, regards social structures as synchronic, with the social structure at time t being a product of the mental states of individuals at time t (see  section 4.1).  John Searle’s theory of institutional facts (Searle 1995, 2010) also regards social entities as being synchronically dependent: the institutional facts at time t are a product of attitudes in the community at time t together with a synchronic residue of historical events that Searle calls the “background”. Work in a variety of domains, however, argues for an ontological role of historical factors. Among these are theories of semantic content (Kripke 1972, Putnam 1975, Davidson 1987), biological and social kinds (Millikan 1984), artworks (Levinson 1980), and artifacts (Bloom 1996, Thomasson 2003).