Bob Brown's speech to the National Press Club yesterday was an unusual mix of economic xenophobia and social cosmopolitanism, including support for a world parliament:

Why shouldn't we now join vigorous moves in Europe and at the United Nations for a global people's assembly based on one person, one vote, one value? Such a global parliament — it could be right here in Australia — would tackle international questions like nuclear proliferation, currency speculation, marine ecosystem destruction and those billion people who could be fed and literate if only a tenth of global military spending was sent to their assistance.

The instant response from many was to laugh, but looking at the main arguments against (it's irrational, it's a-historical and it won't work), it's not a foolish idea.

On pure rationality alone, the idea of a parliament of the people of one planet makes sense. There's no real logical reason why democracy ought to be confined to national borders. Not when states come in such varying sizes (from a few thousand in Nauru to 1.3 billion in China), and when their borders only tangentially relate to how people identify themselves, or the globe's geography.

Likewise, many of the problems we face (financial markets, food and water, terrorism, refugees, climate change) have little respect for human-drawn lines on a map. Our governance must rise to the level of the problems we face.

If the idea is not irrational, many will argue it's a-historical, but that seems wrong too. Most of human history has been lived outside the control of states and many still don't see states as the best institutions to govern them. I imagine that, for those in the late 19th century, structures such as the UN would seem a-historical, but today it's commonplace.

The final argument is that it won't work, and while I don't discount the immense challenges, the pessimism of our recent multilateralism debate shouldn't control our thinking. Multilateralism as it currently stands is much like the House of Lords a century ago: you need to be an elite with vast interests to get in, and the size of your resources largely dictates your influence. A world parliament based purely on population could escape that.

One way to gain support for the idea would be to follow Bob Carr's suggestion and require countries to be democracies before their citizens could participate. The new institution would almost have a majority of the world's population, avoid the 'rewarding dictatorship' problem of the UN, and provide a powerful incentive for people around the world to move their own states to a democracy.

I'm not saying a world parliament should become a cause for Australian foreign policy any time soon. But it's an idea we should treat seriously, not laugh at. For a longer discussion of the idea, I recommend George Monboit's book, The Age of Consent.