Sama> How should someone figure out how they should be useful?
Elon> Whatever this thing is you are trying to create.. What would be the utility delta compared to the current state of the art times how many people it would affect?
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Let me make my point in a less obtuse way. Most people make decisions about their careers based on opportunity and maximising profit. No one becomes a footballer to make the world a better place. This would all be fine as long as the capitalist market rewarded choices that make the world a better place. Obviously it does not and it's not the fault of a footballer that we as a civilisation choose to channel our available resources their way and not towards frivolous play like space exploration.
If anyone ever figures out a way to make the free market choose the greater good they will win all the Nobel prizes forever (we won't need Nobel prizes after that).
But to address your argument: you missed the "opportunity" bit. If you're a well-off, well educated person you can choose to do whatever you feel like and I'm sure some people do without worrying about money (perhaps they have lots already). Most people will chose the best paying job that is available to them. Sorry, but reality is on my side on this one. Go visit a factory if you want to see if people want self-realisation or money to survive. Again: please remember the "opportunity" part. If you tell a factory worker he can earn 4$ an hour assembling landmines or 2$ an hour assembling asthma inhalers I will eat my hat if they don't go with the money (and I wouldn't blame them for this).