On the Equivalence between Neyman Orthogonality and Pathwise Differentiability

It has been frequently observed that Neyman orthogonality, the central device underlying double/debiased machine learning (Chernozhukov et al., 2018), and pathwise differentiability, a cornerstone concept from semiparametric theory, often lead to the same debiased estimators in practice. Despite the widespread adoption of both ideas, the precise nature of this equivalence has remained elusive, with the two concepts having been developed in largely separate traditions. In this work, we revisit the semiparametric framework of van der Laan and Robins (2003) and identify an implicit regularity assumption on the relationship between target and nuisance parameters -- a local product structure -- that allows us to establish a formal equivalence between Neyman orthogonality and pathwise differentiability. We demonstrate that the two directions of this equivalence impose fundamentally different structural requirements, and illustrate the theory through a concrete example of estimating the average treatment effect. This helps clarify the relationship between these two foundational frameworks and provides a useful reference for practitioners working at their intersection.