Both a Start & a Full are good-a thought

I had an interesting discussion with a non-writer last night. I mentioned that sometimes a story, even novel-length, will appear to me completely done, from start to finish. He protested that one should write without knowing at the beginning what will happen, like life. Why one or the other? If you have only a start, start it. If you have the whole thing, complete it. 

JRMcF