> The best possible essay at any given time would usually be one describing the most important scientific or technological discovery
To a hammer, everything is a nail. I love technology and science, but that doesn't mean that is what makes the best essay. Relationships are key to our survival, so I suggest personal relations could be just as good an essay as anything scientific - yes, I'm suggesting that social science isn't really a science.
This also then leads to timelessness. The essay on natural selection is timeless. It wouldn't be written today, but it is still a viewport into the discovery, and of the time, and the knowledge is still valid. Maybe I am misunderstanding the point about timelessness.
> The other reason the initial question matters is that you usually feel somewhat obliged to stick to it.
I also disagree with this comment. I'm currently writing a talk I've been asked to present, and in the process of my writing and researching, I've discovered a whole new and better question. I find the point of a good question is that it can lead to better questions. I do feel that PG is suggesting this as well, but maybe this one sentence just stuck out to me.