[pg]

Your mistake in turn is to assume that it's impossible for people to judge their own abilities.

Lots of people think they could write a decent novel. Most are wrong. Suppose only .01% actually could. If your argument were correct, JK Rowling should assume her chances of writing a decent novel are .01%. She feels fairly confident that she could, but she has to discount that, because people are often mistaken about such things, and "fall back to the raw figures."

It is possible to know that one is good at something.

[EGreg]

Not only whether one is good at something, but whether they are in a good position to try. If you already have lots of traction, or investors, or smart people around you, then it's a good opportunity.

Having said that, what were the people who funded Color thinking?

[Negitivefrags]

That is not an example of JK Rowling judging her own abilities. Her abilities were judged by the market when she published her books.

It's possible to know one is good at something, it's just not possible to judge your own abilities.

[pg]

I'm not talking about an existing book. I'm talking about the case where she's trying to predict the chance she'll be able to write a new book, before she's started it.

Her previous books should give her confidence that she'll be able to write another. That is one way in which she can judge her own abilities.