Every semester I teach a studio on interior lighting. By the end of it, I am always more uncertain than when I began — and I consider this a sign that the semester has gone well.
My students ask questions I do not expect. This year, one asked: "How do you know when a lamp is finished?" I had no answer that satisfied either of us. We talked about it for an hour.
The best answer I have found since: a lamp is finished when you stop looking at it and start seeing what it illuminates.