 In classical logic, the domain for quantification is assumed to be non-empty and every individual constant is assumed to refer to something in the domain. In free logic, neither of these assumptions is made. Thus free logic appears to be particularly suited to theorizing about non-existent objects; see Lambert 1991, Jacquette 1996. For a criticism of the free-logical approach to fictional discourse, see Woods 1974: 68–91. Interestingly, the Barcan Formula and the Converse Barcan Formula are not derivable in free logic.