 How innocent logic is with respect to ontology is also at the heart of the debate about the status of second order logic as logic. (Quine 1970) argued that second order logic was “set theory in sheep’s clothing”, and thus not properly logic at all. Quine was concerned with the questions of whether second order quantifiers should be understood as ranging over properties or over sets of individuals. The former were considered dubious in various ways, the latter turn second order logic into set theory. This approach to second order logic has been extensively criticized by various authors, most notably George Boolos, who in a series of papers, collected in part I of (Boolos 1998), attempted to vindicate second order logic, and to propose a plural interpretation, which is discussed in the article on  plural quantification.