Grounding, Explanation, and the Limit of Internality ∗ Jon Erling Litland [This paper is due to appear in Philosophical Review. It may be subject to minor changes. The authoritative version should be obtain from Philosophical Review] Introduction This paper is about the interaction of grounding and metaphysical modality. It begins negatively. Most philosophers writing on grounding have held that grounding is in an internal relation in the following sense: if φ grounds ψ it is necessary that if φ and ψ both obtain then φ grounds ψ. But this is not so. Establishing this is the burden of the first, negative, part of this paper. While there has already been some dissent the present argument is very different from the ones already in the literature: I derive a contradiction from the assumption that grounding is internal in the above sense, relying ∗ Material from this paper has been presented at the csmn Colloquium (), at a workshop on grounding in Gothenburg (), and at the Recent Work on the Logic of Ground Workshop in Oslo () as well as at the Central APA. Thanks to the audiences at those occasions. Thanks to Øystein Linnebo, Kit Fine, Shamik Dasgupta, Louis deRosset, Kelly Trogdon, Tim Button, and Sam Roberts. Thanks especially to Jönne Kriener, Michael Raven, and Louis deRosset for detailed comments on earlier drafts. Thanks also to an anonymous reviewer for helpful advice. A non-exhaustive list is Rosen ; Bennett ; Dasgupta , forthcoming(a),(b); deRosset , –, ; Fine b; Trogdon b. See for instance Leuenberger ; Chudnoff n.d.; Skiles forthcoming. Due to lack of space I will not be able to discuss these other views. Schaffer (, –) is also an exception, but irrelevantly so. His reason for denying that grounding is an internal relation