The Muses

"There is a third form of possesion or madness, of which the Muses are the source. This seizes a tender, virgin soul and stimulates it to rapt, passionate expression, especially in lyric poetry, glorifying the countless mighty deeds of ancient times for the instruction of posterity. But if any man comes to the gates of poetry without the madness of the Muses, persuaded that skill alone will make him a good poet, then shall he and his works of sanity with him be brought to nought by the poetry of madness, and behold, their place is nowhere to be found."

- Socrates (from the dialogue Phaedrus)

Edit - In case you are wondering, that image is Calliope, the Muse of beautiful words.