This dissertation is about what is called a conception of scientific explanation. A conception of scientific explanation concerns some ontological questions about scientific explanation. Two related but distinct questions have been investigated: (1 What type of entity is an explanans? (2 In virtue of what, is a thing of that type explanatory?There are two competing conceptions of scientific explanation. One is the ontic. conception and the other is the non-ontic conception. A conception is ontic if it says that (a a type of thing that explains is primarily a thing in the world that is responsible for a target of explanation (ontic explanation and (ba type of entiry explains if and only if it is either ontic explanation or accurately represents the relevant ontic explanation (ontre determmation. A conception is non-oane if it denies either (a or (b or both in one way or the other