 Moreover, philosophers have questioned whether Frege’s Rule of Substitution (Grundgesetze I, 1893, §48, item 9) also required an appeal to intuition. The Rule of Substitution allows one to substitute complex formulas for free second-order variables in logical theorems to produce new logical theorems. Boolos argued (1985, 336–338) that since Frege’s Rule of Substitution is equivalent to a Comprehension Principle for Concepts, it is extralogical in character. The Comprehension Principle for Concepts asserts \(\exists F \forall x(Fx \equiv \phi)\), provided \(\phi\) doesn’t have a free variable \(F\); the proviso blocks the instance \(\exists F\forall x(Fx \equiv \neg Fx)\), from which one can quickly derive a contradiction. Thus, the Comprehension Principle for Concepts asserts the existence of a concept corresponding to every expressible condition on objects. From Kant’s point of view, such existence claims were thought to be synthetic and in need of justification by the faculty of intuition. So, although it was one of Frege’s goals to avoid appeals to the faculty of intuition, there is a question as to whether his system of second-order logic (minus Basic Law V), which implies a principle asserting the existence of a wide range of concepts, really is limited in its scope to purely logical laws of an analytic nature.