So here I am, in the middle way, having had twenty years— Twenty years largely wasted, the years of l'entre deux guerres— (332-333)
The speaker reflects on the writing career he's had leading up to "Four Quartets." In a gesture of humility, he admits that these twenty years have been largely wasted. Why? Well because the twenty years have taken place between two World Wars, and no matter how much he's tried to write the truth, humanity is still prepared to make all the same mistakes it made twenty years earlier. No progress has happened.
It seems, as one becomes older, That the past has another pattern, and ceases to be a mere sequence— Or even development: the latter a partial fallacy Encouraged by superficial notions of evolution (476-479)
As you get older, you start to realize that the idea of human progress is a little bit farfetched. If anything, it starts to look as if things have steadily gotten worse for humanity throughout history. Sure, we don't die from the plague anymore. Instead, we die in wars that wipe out tens of millions of people in only a few years. The idea of evolution in this sense is pretty silly, since it's really tough to argue that human beings are becoming stronger or better as time goes on.
We had the experience but missed the meaning, And approach to the meaning restores the experience In a different form, beyond any meaning We can assign to happiness. (483-487)
In the past, we might have had the kind of spiritual experience that the speaker wants us to have, but we missed the meaning of this experience because we don't really know what to look for. That said, all is not lost. If we can somehow approach the meaning that this spiritual experience was supposed to have, we can still recover the truth that we missed the first time around. This won't necessarily be a happy experience, because true learning rarely is.
I have said before That the past experience revived in the meaning Is not the experience of one life only But of many generations—not forgetting. (487-490)
The past contained in our memory isn't personal, you know. Our memories form part of the history of an entire culture, and of many generations stretching back into the past. We can never truly know how much the past is governing what we do, but it's totally silly to think that we're a bunch of individuals whose personal memories are starting from point zero when we're born.
The backward look behind the assurance Of recorded history, the backward half-look Over the shoulder, towards the primitive terror. (492-494)
We might be pretty comfortable with the idea that humanity is making progress through history. After all, our history textbooks prove this, don't they? Well, behind these comforting textbooks ("the assurance of recorded history"), we might actually glimpse a terrible truth: that humanity hasn't learned from its mistakes at all, and that we modern folks are not any more intelligent or moral than all of the people who came before us. We have to come to terms with this fact before we can start really getting a grip on how to improve ourselves and the world.
We shall not cease from our exploration And the end of all our exploring Will be to arrive where we started And know the place for the first time. (866-869)
We might think we've seen it all and done it all, but the truth of life isn't to know everything. It's to take a close look at what you think you know and to see it again for the first time. In this case, we need to get over the confidence we have in the thought that we already know a lot, and we need to realize that life isn't about getting anywhere, per se, but about getting back (for once) to where you already are. This is how you start living in the "here" and "now."