Start reading a journal you wrote a decade or two ago, and within a few pages, you'll find ideas you could have sworn you only came up with in the last few days or weeks, albeit slightly differently formulated.
You dig yourself into a rut. It becomes your world, a riverbed that your thoughts rarely overflow.
Didn't Ezra Pound say somewhere that, as we get older, we discover more and more how true were the things we believed when we were young? This probably has to do with selective perception. Darwin wrote in his autobiography that he found it necessary to write down every piece of evidence which appeared to contradict his beliefs, since otherwise he knew they would disappear from his mind.
“As I'm sure you know, water always picks the shortest route to flow down. Sometimes, though, the shortest route is actually formed by the water. The human thought process is a lot like that.” -- Haruki Murakami, “Where I’m Likely To Find It.”
There is something to what you say. Diversity of experience helps: moving among different careers so you always have to learn something totally different. I find this generates my best truly new ideas. I am in total accord with the dramatic decrease in the flux of *new* ideas from about age 25 and up. Good thing I wrote most of them down back in the dya.
I'm gonna have to correct Haruki, though. The Principle of Least Action is perhaps the most fundamental idea for how nature behaves. It's uncanny. In words: the action is the sum of the difference in the kinetic and potential energies as it evolves. In concept: the universe is lazy. Things like to be in the lowest state of energy they can find and they like to be all disordered and messy. Water doesn't pick the shortest route (c.f. the Colorado River), it picks the route with the lowest action. If we go all abstract (which we just love to do!) then we can map the trajectory of the river into a subspace so that the least action is also the "shortest" route and call it a geodesic.
I love posts like this – and I have to agree (with Ransom too) – youth, without the benefit of experiences, provides the most ideas. It is striking to think about. I found though that the ideas I had when younger were simply beyond me to explore fully (at the time). I take those younger ideas now and give them all sorts of twists and, hopefully, insight. Or just give them away. Sometimes the point of having an idea is learning to let go of it.
My old journals (they go back to age 9) read like they belong to a different person. I find that I don't even remember reading more than half of the books I mention in my old journals. I can hardly imagine the thought process behind half of the entries. The ones that I can imagine thinking (from a year or two ago), I don't want to think anymore. I'm tempted to change my mind about something simply based on the fact that I had thought like that a year ago.
My old journals read like they belong to an entirely different person too — yet a person whose ideas are inexplicably the same ideas I came up with last week. And even if I do change my mind every year, plus ca change, plus c'est la meme chose.
Letting go of ideas… I need to get better at that. Trying to restate Murakami's observation in line with Ransom's corrections, I come up with the following:
"Water always follows the path of least resistance. The path of least resistance is never formed by the water. The human thought process is a lot like that."
I actually suspect the human thought process is a lot like that… however Ransom cautions me that if there is A LOT of water then "charge will arc or spark to get over a high resistance hurdle to a much lower resistance path than it could at a lower potential… if that makes any sense."
From Robert Musil's The Man Without Qualities — “If a young man is intellectually alive… he is continually sending out ideas in all directions. But only what produces resonance in his environment will radiate back to him and condense, whereas all the other messages are scattered in space and lost… in the course of time one's ordinary and non-personal ideas intensify quite of their own accord and the extraordinary ones fade, so that almost every one of us steadily becomes more mediocre, with all the certainty of a mechanical process…”