 However, the relations corresponding to such predicates are strictly weaker than PP and PE and no biconditional is provable in M that would yield a corresponding definiens of ‘P’ (though one could define ‘P’ in terms of ‘O’ or ‘D’ in the presence of further axioms; see below ad (61)). Thus, other things being equal, ‘P’, ‘PP’, and ‘PE’ appear to be the only reasonable options. Here we shall stick to ‘P’, referring to J. Parsons (2014) for further discussion.