Are there certain intangibles that every entrepreneur simply cannot be successful without? PG has cited determination and toughness as two essential traits that

Are there certain intangibles that every entrepreneur simply cannot be successful without? PG has cited determination and toughness as two essential traits that YC looks for in entrepreneurs. Vivek seems to argue that there aren't any traits to you HAVE to have, but that most people, given the right opportunity, can learn and grow into the right person to create a successful company. I don't agree with this, but I do think it has some merit in that you can't really tell how well someone will stand and deliver until you push them into a wall, which you can't really do in any kind of formal interview process or pitch. So how can you tell how determined or tough someone is?