It's acceptable provided that I can accurately measure the outcome. So, I would be willing to e.g. participate in a business venture with payoff (if succesful) of x and known probability p of succeeding provided p*x is better than any other options I have; however, if p is highly uncertain, then I'm wary of it -- I don't trust my judgement, I think it's too easy to be overconfident. I think I could develop a better attitude about risk, however.