There's something like a midwit peak for making money. If you don't need to make much, you can work on whatever you're most interested in; if you want to become moderately rich, you can't usually afford to; but if you want to become super rich, and you're young and good at technology, working on what you're most interested in becomes a good idea again.

Many if not most of the biggest startups began as projects the founders were doing for fun. Apple, Google, and Facebook all began that way. The best ideas tend to be such outliers that you'd overlook them if you were consciously looking for ways to make money.

The odds are better when you have strange tastes: when you like something that pays well and that few other people like. Bill Gates didn't just love programming — he loved writing software for customers. That is a very strange taste indeed, but if you have it, you can make a lot by indulging it.

When you can't decide which path to take, it's almost always due to ignorance. You're usually suffering from three kinds of ignorance simultaneously: you don't know what makes you happy, what the various kinds of work are really like, or how well you could do them. The cure is to start doing things, not to deliberate longer.

Stay upwind. The less sure you are about what to do, the more important it is to choose options that give you more options later. If you're unsure whether to major in math or economics, choose math — it's easier to switch from math to economics than the reverse.

You'll become like whoever you work with. If you choose work mainly for how well it pays, you'll be surrounded by others who did the same, which makes it even more soul-sucking than it looks from outside. Choose interest and you'll be surrounded by the genuinely interested.

PG's heuristic for evaluating a kind of work: look at who your colleagues will be.

A test for whether someone actually loves their work: if it didn't pay well, would they take a day job doing something else in order to do it in their spare time? Lots of mathematicians, scientists, and engineers would. Not as many investment bankers would.