 …physical theory only characterizes its basic entities relationally, in terms of their causal and other relations to other entities. Basic particles, for instance, are largely characterized in terms of their propensity to interact with other particles. Their mass and charge is specified, to be sure, but all that a specification of mass ultimately comes to is a propensity to be accelerated by certain forces, and so on. Each entity is characterized by its relation to other entities, and so on forever. …The picture of the physical world that this yields is that of a giant causal flux, but the picture tells us nothing about what all this causation relates. Reference to the proton is fixed as the thing that causes interactions of a certain kind, that combines in certain ways with other entities, and so on; but what is the thing that is doing the causing and combining? As Russell (1927a) notes, this is a matter about which physical theory is silent. (Chalmers 1996: 153)