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Physics-guided machine learning (PGML) has emerged as a hybrid paradigm in materials science, integrating domain knowledge with data-driven methods to enhance predictive accuracy and generalizability. Conventional approaches typically embed physical principles as soft inputs—either through loss-function regularization or auxiliary features—allowing violations during optimization. This manuscript advances a conceptual reframing in which physics operates as a hard constraint on the model’s hypothesis space rather than as an additive input. By restricting permissible functional forms, symmetries, and conservation relations a priori, the framework enforces physical consistency at the architectural level, altering the interaction dynamics between data and prior knowledge. The reframing yields systems-level insights into epistemic trade-offs: reduced reliance on large datasets, improved extrapolation beyond training regimes, and inherent satisfaction of thermodynamic or mechanical invariants critical to materials behavior. Analytical implications include feedback structures that couple data refinement to constraint satisfaction, revealing emergent robustness in multiscale modeling. This perspective addresses persistent challenges in materials science, such as sparse experimental data and complex microstructure-property relationships, without resorting to empirical validation. The contribution lies in reinterpreting PGML’s epistemic foundation, steering future developments toward constraint-centric designs that prioritize physical fidelity over post-hoc penalization.