 One major issue in the metaphysics of causation concerns specifying the relata of causal relations.  Consider a mundane claim: an iceberg caused the Titanic to sink.  Does the causal relation hold between two events: the event of the ship hitting the iceberg and the event of the ship sinking? Or does it hold between two sets of states of affairs? Or does it hold between two substances, the iceberg and the ship? Must causal relations be triadic or otherwise poly-adic? For example, one might think that we are always required to qualify a causal claim: the iceberg, rather than the captain's negligence, was causally responsible for the ship’s foundering. And can absences feature in causal relations? For example, does it make sense to claim that a lack of lifeboats was the cause of a third-class passenger's death?