This is very good advice, even though 95% of the people who hear it will still think they're special and don't need to learn their craft before jumping into cool stuff.
The thing I always remember from reading Feynman's various autobiographical essays -- he would always try to figure out something from first principles.
If you don't know the 'simple, always true' laws of what you're working with, you're going to be afloat on a sea of conjecture and fuzzy thinking.
I can't upvote this enough. "From first principles" is the only way to master something -- math, programming, automotive maintenance, whatever.
There aren't ever that many foundational principles to a field of a study, but if you understand them -- really understand, until it becomes intuitive -- you're 80% the way to being an expert.
The thing I always remember from reading Feynman's various autobiographical essays -- he would always try to figure out something from first principles.
If you don't know the 'simple, always true' laws of what you're working with, you're going to be afloat on a sea of conjecture and fuzzy thinking.