Descend lower, descend only Into the world of perpetual solitude, World not world, but that which is not world, Internal darkness, deprivation. (117-120)
The speaker wants us all to find our inner spiritual light; but in order to do this, he pretty much says we'll have to totally strip down everything we know and love about ourselves. We have to go into a realm of "Internal darkness, deprivation" before we can come out on the side of spiritual fulfillment. Sheesh, maybe that's why Eliot's work is usually so dreary.
O dark dark dark. They all go into the dark, The vacant interstellar spaces, the vacant into the vacant, The captains, merchant bankers, eminent men of letters. (280-283)
Who's going to die eventually? The answer is everybody, no matter how rich or famous they become. It doesn't matter if you're a captain, merchant banker, or (like T.S. Eliot) an eminent man of letters. Everyone dies eventually, and the sooner we all accept that we're all alike in this way, the sooner we can get down to the business of leading better, more humble lives.
In order to arrive at what you do not know You must go by a way which is the way of ignorance (318-319)
Spiritual fulfillment is something we haven't found yet, and if we plan on achieving it, we're not going to get there by trying to "master" our spirits. We can't just force enlightenment on ourselves. We have to adopt and attitude of submission, of passivity. We have to become humble and allow something beyond ourselves to take over. This might be God or it might be nature. The most important part of this passage, though, is that we have to admit our own ignorance before we can start to learn.
From wrong to wrong the exasperated spirit Proceeds, unless restored by that refining fire Where you must move in measure, like a dancer. (771-773)
The speaker shows us that fire doesn't just have to be a symbol of ambition or lust. Rather, it can be an image of purification and humility. Like fire, the lesson of humility can really cause us a lot of pain. But also like fire, humility can purify us of all our bad qualities and make us better in the long run. If we don't learn to accept the pain of humility, then we'll just spend our whole lives moving from bad decision to bad decision ("From wrong to wrong"), getting more exhausted as we go.
Quick now, here now, always— A condition of complete simplicity (Costing not less than everything) (879-881)
If we're ever going to get out of our own heads and start appreciating the world around us, we're going to have to start focusing on the "here" and "now" in a totally uncomplicated way. The thing is, though, that if we're going to do this, it's going to cost us everything, especially our sense of individual pride.
Who then devised the torment? Love. Love is the unfamiliar name Behind the hands that wove The intolerable shirt of flame Which human power cannot remove. We only live, only suspire Consumed by either fire or fire (834-840).
So why does it hurt so much to be humble? What is responsible for all the pain we're willing to endure to become better people? In short, the answer is love. Love is what makes us put on the "shirt of flame" and live through the pain of being humbled. Without love, we would never try to improve ourselves. There's just no choice in the end. If we want to become better people, we have to become more humble, and either way, the experience is going to be painful.