This Sea of Error

"Happy the man who still can hope

to swim to safety in this sea of error.

What we don't know is what we really need,

and what we know fulfills no need at all."

-- Lines 1064-1067

Are these lines at the heart of Faust's discontent? Is the problem with his books that they lack a guide through their "sea of error"? Is this why Faust summons Mephistopheles - is it better to have the devil for a guide than no guide at all? "The way up is the way down"... remember Four Quartets and the speaker's descent into darkness? Is Faust actually on the way to salvation when he calls Mephistopheles?

Also, another question (but maybe one we can't answer yet). Mephistopheles has a lot of human qualities - he looks like a human, talks like a human, has humor and sarcasm, and seems to inhabit a similar middle ground (above animals, below the divine). Can we conclude anything about humanity from this? In particular, is Goethe implying that evil is a part of human nature, not some oustide force?