I have a question about the expression "who is to say." Is it interchangeable with "who could say"?

LIke, "who is to say what's beautiful" and "who could say for sure what's beautiful." Are these close enough?

mitsuwao23 "who is to say." Is it interchangeable with "who could say"?
I wouldn't exactly use them interchangeably, although some people might in certain situations.

"Who is to say?" questions our authority. (No individual should make the specified statement.)

"Who can/could say?" questions our ability to answer. (Nobody is able to say for sure.)

"Who is to say what's beautiful?" is fine. (Beauty is in the eye of the beholder.)
"Who can say what beauty is?" This is a different question.

Each of these questions may or may not be answerable. (Of course they're both intended as rhetorical questions.)

Who is to say that we shouldn't form our own company?

Who can say where we'll all be ten years from now?
Ahh, authorities and abilities. Good point. Now I am sure they are different.

Thanks again for your clear explanation.