But does that mean you need a skill system, with a list of discrete skills?
I am perfectly content with using background/class: "I was a pirate, so I probably know something about sailing."
I get that some people really enjoy detailed skill systems, and investing variable numbers of points, etc. I'm just saying that it isn't actually needed to differentiate trained vs. untrained. Granularity is only a preference.
EDIT: I have a game somewhere but I honestly can't remember the name...it's designed to introduce RPGs, and is kid-friendly, with kind of basic cartoony art...where character creation is just "allocate 10 points to any concept you want". There's no list; you just make up whatever class/background/profession/hobby you like. So I could put 5 into pirate, 3 into dancer, and 2 into horticulturist. If I can explain why the background would help me with a task, I get to roll a number of d6's equal to the points I allocated. Never actually played it, but I kinda love the idea. "Oh, I'm a pirate, and pirates have to sew and mend their own clothes, so...."