Accepting Newell's invitation to develop his interdisciplinary theory to address new developments, I first consider Newell's complex systems theory of interdisciplinarity. I then review developments of his theory before critiquing its merits and deficiencies. This critique is informed by recent critical realists' works that point to deficiencies rooted in interdisciplinary theory's holistic realist ontology, which they argue is incomplete. I then review critical realists' ontology and their case for interdisciplinary epistemology, which I argue marginalizes constructivism. Finally, I outline a constructivist-realist ontology. This approach incorporates an agentic theory of human agency and a social network view of social structure. I then consider the implications of this analysis for Newell's original ontology, and thus for his interdisciplinary epistemology.