This paper proposes a reconceptualization of the human subject for the computational era through the integration of relational ontology and coevolutionary dynamics. Its central thesis is that relational coherence constitutes the fundamental organizing principle of complex systems and that this principle requires a transition from Homo Rationalis—the isolated, utility-maximizing individual inherited from Enlightenment thought—to Homo Interaction, a relational subject whose identity, cognition, and agency emerge through dynamic alignment with others. The paper’s principal theoretical contribution is the formalization of a Dynamic Law of Knowledge Expansion, which models the rate of knowledge generation as a function of relational coherence, plurality, and cognitive energy. Relational coherence is quantified through the Kuramoto order parameter, plurality through phase entropy, and stabilized knowledge as the cumulative persistence of coherent attractors. The resulting equation demonstrates that knowledge creation is maximized at intermediate levels of plurality, thereby formally embedding the Coherence Paradox: excessive synchronization suppresses the diversity required for long-term adaptation and innovation. This multiscale formulation is derived from underlying phase dynamics and establishes a mathematically connected framework linking topology, synchronization, entropy, and epistemic growth. The Coherence Paradox is examined as a universal structural principle operating across biological evolution, reinforcement learning, scientific communities, economic systems, and cultural dynamics. Four falsifiable predictions are proposed to enable empirical evaluation of the model. The paper also introduces two speculative design frameworks—wearable coherence interfaces and the IA Deus Stack—as prospective architectures for measuring and navigating relational coherence in human–AI systems. The study concludes that Homo Interaction is not a distant possibility but an emergent condition already unfolding through the coevolution of humans and artificial intelligence. The fundamental challenge is not whether this transition will occur, but whether humanity can assume ethical and systemic responsibility for the relational architectures it is creating.