Recently, a theoretical framework for nanoparticle transport in the laminar flow regime has been proposed. The theory features a rigorous gas-kinetic theory analysis. It considers the effect of nonrigid body collision, and the theory is shown to reproduce the Chapman-Enskog theory of molecular transport in the small particle size limit, Epstein's model of particle drag in the rigid-body limit, and the Stokes-Cunningham equation for the drag on micrometer size particles. This theoretical framework provides the hope that bits and pieces of particle transport theories formulated over the last century can now be unified into a generalized theory. This paper discusses an unresolved fundamental issue related to this generalized theory, namely, the transition from specular scattering applicable to molecule-molecule collision to diffuse scattering governing molecule-"particle" collision.