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