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).
I certainly did not chose my profession from a profit motive and neither did most of the people I know. We did what we did because that is what we like or enjoy. Of course practical issues of making a living wage factors in.
I make a high salary now, but that is rather by accident. I just happened to be good in STEM subjects and enjoy math and programming. I didn't pursue it because it was an optimal economic decision.
I can fully relate to Elon Musk. When I try to pick a programming job e.g. I factor in many things such as salary, colleagues, location etc, but actually how useful the product seems is a major factor for me. I am generally willing to sacrifice salary to do something which I feel helps humanity more. E.g. I'd day a pay cut to develop a medical application over a horse betting application.
Of course money isn't irrelevant. If there is just too little money in medical software then I'd suck it up and sell my soul to betting, big oil or whatever ;-)