One of the strongest arguments against Brandom's inferentialist justification of modus ponens is the concern that it fails to fully account for the modal force and necessary truth of logical laws like modus ponens.

While Brandom's approach elegantly grounds modus ponens in the implicit norms and inferential commitments we undertake by using the conditional connective in our linguistic practices, this still seems to fall short of fully capturing the modal metaphysical aspect of why modus ponens must hold across all possible worlds and interpretations.

The validity of modus ponens appears to go beyond just patterns of inference we contingently engage in. There is a necessary, universal character to its truth that seems to demand a deeper justification than just being built into the conceptual roles and meaning-conferring practices we adopt.

Brandom may be correct that grasping the meaning of the conditional involves committing to drawing the modus ponens inference. But one could argue that a more robust account is still needed for why that particular inferential commitment is necessarily truth-preserving and metaphysically universal across any possible state of affairs.

The intuitive force and modal robustness of logical laws like modus ponens suggests there are objective modal constraints on reasoning that may transcend the conceptual role semantics and implicit normative attitudes that Brandom emphasizes. A fuller grounding in metaphysical modality may be required to completely justify modus ponens and capture its modal força.