Great minds have said of me that:

I am moonlight in the gloomy night of life.
I can soften rocks or bend a knotted oak.
Without me, life would be an error.
I separate the generations.
I express that which is inexpressible
And on which silence is impossible.
I am the art most nigh to tears and memory.
I am the shorthand of emotion.
I should strike fire from the heart of man,
And a woman I should bring to tears.
I can name the unnameable and communicate the unknowable.
I have been called the speech of angels.

What am I?