I must say, First Minister, that I feel a very deep emotional attachment to Britishness and to Welshness, and I do believe the union is much more than a constitutional arrangement. So, that's the first remark I'd make about the attitude you're taking to undoubtedly needed constitutional reform.

I do welcome reforming our union. I think it is an interesting document and I give it two cheers, broadly. But there's a very big flaw in it and that's the handling of the concept of sovereignty. Sovereignty in terms of conceiving political associations is the trickiest concept in political philosophy, but it does require really deep thought, and we can't avoid doing this because it has very practical consequences. We see that in much of the Brexit argument at the moment. So, I think when you say in the document that

'it is accepted that sovereignty (some of which should be shared) lies with each part of the UK',

I think you need to be really careful about syntax here. It lies 'in' each part of the UK, I don't have a problem with that, because I do think our institutions should be embedded, but it also is embedded at the UK level and that's a crucial distinction.

I note that your distinguished predecessor wrote last week:

'End Westminster sovereignty and give it to the four nations.'

From which, presumably, some form of pooling for defence, foreign affairs, macroeconomic policy and the like takes place. But, I cannot support this concept because it is not a federal principle. It is a mistake to see federalism as characterised by such radical decentralisation. Rather, federalism seeks to create a central and effective sovereign authority, albeit one limited in scope.

It is sometimes said that Alexander Hamilton accepted federalism as the price to create a central government. They were, at the time of the US constitution being ratified of course, suffering from the anarchy they'd had in the articles of confederation. So, that was the nature of federalism: to create a central authority that had the power to act.

However, I do believe that these ideas could be tested and examined in a constitutional convention and for over 10 years I've thought this necessary. And I do note with approval the Welsh Government's call for the principle of subsidiarity to be primary in a written constitution. I think that does lie at the heart of federalism.

Finally, House of Lords reform is surely overdue. It's been imminent, as the joke goes, since 1911, and surely it should become the House of the Union. But let me finish on this issue of sovereignty. Popular sovereignty I think is, when used loosely, a very dangerous concept. Sovereignty certainly is drawn from the people, but it is nearly always expressed in political institutions. There are occasional exceptions to that like referendums. And I think that we need a lot of care and precision when we're looking at the fundamentals of our constitution, but this debate has to be had, and it's well begun.