Life as we experience/comprehend it is merely a series of illusions?
Existing and living are two separate things.
We create our own reality/illusions.
And also as iwamoto already put it, but with a more cynical twist.
Meaning that your hopes and dreams are illusions,
never really obtainable, but needed to sustain you regardless.
Or at least that is how I would read it at the moment.
This is, of course, subject to change.
I think Twain was particularly connected to the Creative Spirit.
He seems to be saying that giving up illusion and only subsisting on the cold hard fact of reality is like losing your soul.
You will still continue to drift through the days, but all hope and spirituality would be gone from your life and it would be worthless.
I like this quote, I’ll try and look it up after i get done working