 Boolos (1985) noted that the Rule of Substitution causes a problem of this kind for Frege’s program given that it is equivalent the Comprehension Principle for Concepts. Boolos suggests a defense for Frege with respect to this particular aspect of his logic, namely, to reinterpret (by paraphrasing) the second-order quantifiers so as to avoid commitment to concepts. (See Boolos (1985) for the details.) Boolos’s suggestion, however, is one which would require Frege to abandon his realist theory of concepts. Moreover, although Boolos’s suggestion might lead us to an epistemological justification of the Comprehension Principle for Concepts, it doesn’t do the same for the Comprehension Principle for Relations, for his reinterpretation of the quantifiers works only for the ‘monadic’ quantifiers (i.e., those ranging over concepts having one argument) and thus doesn’t offer a paraphrase for quantification over relational concepts.