I'm no expert in entrepreneurship, but have to believe this is true for the most part. Before leaving, most people probably have a vision of something to start

I'm no expert in entrepreneurship, but have to believe this is true for the most part. Before leaving, most people probably have a vision of something to start on -- but without (1) building the real product, (2) dealing with the details, and (3) interacting with customers, its only a vague vision. Along the way, things will change, people pivot, founders get new ideas, etc. Here's another way to think about it: there is probably a good reason behind YC accepting people without an idea.