> I've been thinking recently about the ratio of time I spend thinking about thoughts other people have had vs thinking about my own thoughts.

I've thought about this, but I thought it was my thought. Without the internet, I could have likely continued walking around thinking it was an original idea.

I think in order to make something yours, even if it's a thought others have had before, you take the whole of something, wrestle with it internally, then pull it apart and put it back together. There will inevitably be parts of the whole that you leave by the wayside, and other parts you stay with. Since you are a person with unique experiences, what remains - when combined with the viewpoints that come from those unique experiences - will effectively be yours.