The answer is: a heavenly muse.
John Milton follows in the tradition of other great philosophers, who, after Parmenides of Elea (who at the beginning of his poem, On Nature, is escorted by the daughters of the Sun, the god Helios, in the pursuit of his difficult journey) raise their voice to the heavens and the gods in order to request their assistance in tackling such a burdensome and difficult enterprise, which is the pursuit of knowledge about difficult, obscure or challenging topics. In the case of Milton, he asks, in his epic poem, Paradise Lost, for the assistance of a heavenly muse in order to explain the problem of evil and suffering and why God would allow them.