 In the later project of constructing matter, from 1914 on, beginning with Our Knowledge of the External World (Russell 1914b), material objects come to be seen as collections of sense data, then of “sensibilia”. Sensibilia are potential objects of sensation, which, when perceived become “sense data” for the perceiver. Influenced by William James, Russell came to defend a neutral monism by which matter and minds were both to be constructed from sensibilia, but in different ways. Intuitively, the sense data occurring as they do “in” a mind, are material to construct that mind, the sense data derived from an object from different points of view to construct that object. Russell saw some support for this in the theory of relativity, and the fundamental importance of frames of reference in the new physics.