Read the passage below regarding customary international law. Then, answer the question by filling in the blank with the exact concept from the text.
The traditional doctrine of customary international law insists that rules binding upon states emerge only from a strict dual requirement: consistent state practice (usus) and a psychological belief that such practice is legally obligatory (opinio juris). For decades, international tribunals meticulously examined centuries of historical archives, diplomatic correspondence, and military protocols to establish this empirical foundation before recognizing a new binding norm. However, the unprecedented proliferation of multilateral treaties in the latter half of the twentieth century prompted a significant methodological shift.
Modern human rights tribunals, driven by the urgency to establish universal protections in a rapidly changing world, frequently bypass exhaustive historical surveys of state behavior. Instead, they increasingly rely on the texts of widely ratified conventions and declarations by international organizations to deduce a nascent legal consensus. Critics of this modern approach argue that elevating aspirational treaty text over concrete state action fundamentally undermines the democratic legitimacy of international law. This modern tendency, they warn, effectively imposes obligations on states based on rhetorical commitments rather than demonstrated sovereign consent.
Yet, defenders maintain that in an era of rapid global interconnectedness, waiting for uniform historical practice is unfeasible. Such resistance, they argue, fails to acknowledge that in the contemporary legal architecture, formal multilateral declarations constitute a modern form of state practice in their own right.
Question:
Based on the logical structure of the first paragraph, the referential phrase "this empirical foundation" serves as a substitute for which specific requirement of customary international law mentioned earlier in the text?