 The second version of the internal-external distinction is favoured by Armstrong (1978b: 84–5, 1989: 43, 1997: 87–9, van Inwagen 1993: 33–4). According to Armstrong, a relation \(R\) is internal iff it’s holding between \(x\) and \(y\) is necessitated by the intrinsic natures, i.e., non-relational properties, of \(x\) and \(y\); otherwise \(R\) is external. The third version is owed to Lewis. Lewis (1986: 62) advanced the view that an internal relation is one that supervenes upon the intrinsic natures of its relata. But Lewis’s definition of “external” is more involved: \(R\) is external iff (1) it fails to supervene upon the nature of the relata taken separately, but (2) it does supervene on the nature of the composite of the relata taken together.