The topics of Grounding and Metaphysical Explanation have been at the forefront of research and debate within metaphysics for the last decade. Grounding is commonly taken to be a relation of non-causal dependence. In this thesis I address the pertinent question as to what sorts of entities are related in instances of grounding. There has so far been little enquiry into this issue, and it therefore requires urgent attention. I argue here that the entities involved in grounding are facts, where facts are true Fregean propositions. True Fregean propositions are abstract entities composed of senses, and are individuated according the senses which they involve. I proceed by setting out some desiderata which the entities involved in grounding will fulfil. Firstly, they will be individuated sufficiently finely as to provide instances of grounding to back all putative cases of non-causal explanation. Secondly, they will ensure that there is unity between instances of grounding and instances of causation. Finally, they will be mind- independent. I survey different types of entity and show that true Fregean propositions are best-equipped to fulfil these desiderata. We therefore have reason to believe that grounding relates facts, so understood. The conclusion of this thesis therefore makes it incumbent upon us to extend our ontology to include true Fregean propositions.

This dissertation has three parts. In "Quantum Entanglement, Bohmian Mechanics, and Humean Supervenience," I defend David Lewis's metaphysical doctrine of Humean supervenience, and traditional metaphysical reductionism more generally, against an alleged holistic threat encapsulated in the non-separability argument from quantum entanglement. I argue that, contrary to popular belief, realism about quantum mechanics is compatible with Humean reductionism. / Philosophy