How do you know if copyright is working?

In my latest Guardian column, "What do we want copyright to do?" I try to get out of the "you're a thief/you're a greedhead" copyright debate by asking what a good copyright system would look like, and suggesting how we might design one:

Let's start by saying that there is only one regulation that would provide everyone who wants to be an artist with a middle-class income. It's a very simple rule: "If you call yourself an artist, the government will pay you £40,000 a year until you stop calling yourself an artist."

Short of this wildly unlikely regulation, full employment in the arts is a beautiful and improbable dream. Certainly, no copyright system can attain this. If copyright is to have winners and losers, then let's start talking about who we want to see winning, and what victory should be.

In my world, copyright's purpose is to encourage the widest participation in culture that we can manage – that is, it should be a system that encourages the most diverse set of creators, creating the most diverse set of works, to reach the most diverse audiences as is practical.

What do we want copyright to do?