1) H&H: no unification is explanation, cause explanation involves acquiring new beliefs, whereas unification consists in the systematization of old beliefs.
2) H&H: Explanation is derivation of the explanandum from a background theory and the auxiliary or contextual information.
3) H&H distinguishes between:
i) the construction of theories and the application of theries; it seems that, they consider that the construction of theories is a less explanatory activity.
ii) specified and unspecified