This chapter extends developments in social ontology to a problem-based account of constitutive constructivism. It characterizes public-interest problems as ontologically subjective objects about which we can make epistemically objective claims. It argues that the ontological structure of public-interest problems defines a principle’s scope of operation and furnishes the materials for constructing a principle’s content and justification. The chapter explains how the ontological structure of public-interest problems supports substantive justification and protagorean objectivity.