 One of the most important differences between Kant and Frege concerns the resources available to logic. Kant’s logic is limited to (a) Aristotelian term logic with a simple theory of disjunctive and hypothetical propositions, and (b) representing inclusion relations among concepts (MacFarlane 2002, 26). By contrast, Frege’s logic includes (a) a term-forming operator \(\sm{\varepsilon}\), which allows one to form the singular term \(\sm{\varepsilon}f(\varepsilon)\) from the function expression \(f(\xi)\), and (b) a Rule of Substitution, which allows one to substitute complex open formulas for free second-order variables in theorems of logic but also allows one to define, and assert the existence of, complex concepts, including concepts defined in terms of quantifiers over concepts.  We’ll discuss both of these resources below, but first, the discussion needs some context.