 This idea was first explored by Mortensen in his 1983, but he abandoned the basic idea as unworkable. Subsequent proposals within the broad program have been offered by Burger and Heidema 1994, Schurz and Weingartner 1987 and 2010, and Gemes 2007. (Gerla 2007 also uses the notion of the relevance of a “test” or factor, but his account is best located more squarely within the likeness approach.)