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What sophisticated ideas did you come up with independently before encountering them in a more formal context?
I'm pretty sure that in my youth I independently came up with rudimentary versions of the anthropic principle and the Problem of Evil. Looking over my Livejournal archive, I was clearly not a fearsome philosophical mind in my late teens, (or now, frankly), so it seems safe to say that these ideas aren't difficult to stumble across.
While discussing this at the most recent London Less Wrong meetup, another attendee claimed to have independently arrived at Pascal's Wager. I've seen a couple of different people speculate that cultural and ideological artefacts are subject to selection and evolutionary pressures without ever themselves having come across memetics as a concept.
I'm still thinking about ideas we come up with that stand to reason. Rather than prime you all with the hazy ideas I have about the sorts of ideas people converge on while armchair-theorising, I'd like to solicit some more examples. What ideas of this sort did you come up with independently, only to discover they were already "a thing"?
When I was first learning about neural networks, I came up with the idea of de-convolutional networks: http://www.matthewzeiler.com/
Also, I think this is not totally uncommon. I think this suggests that there is low-hanging fruit in crowd-sourcing ideas from non-experts.
Another related thing that happens is that I'll be reading a book, and I'll have a question/thought that gets talked about later in the book.