 As we saw in  Section 2.1  Boolos defined an interpretation of the theory MSO of monadic second-order logic in the theory PFO of plural quantification. He sought to use this translation to establish the logicality of MSO. Doing so will require two steps. The first step is to argue that PFO is pure logic, that is, to establish the full Logicality Thesis (however exactly it is interpreted). The second step is to argue that the interpretation of MSO in PFO preserves logicality.