I’M sorry, but I’m going to have to use the F-word: federalism. The idea of federalism floats up from time to time in Scottish constitutional politics. As far as one can tell, Labour is passionately committed to federalism on alternate weekends if it’s not raining.

Gordon Brown was a big fan – although he never actually committed himself to federalism, only to something “as near to federalism as possible in the British system”.

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That is because Gordon Brown, being better informed and perhaps more honest than most, recognised that the British system – if that system is defined by the sovereignty of Parliament – and federalism are utterly incompatible.

You can have federalism, or you can have a sovereign parliament with ultimate authority over all aspects of legislation. You cannot have both.

Federalism is sometimes used as if it were synonymous with ‘‘Devo-Max’’. Federalism, to those who advocate it on those terms, means an extension of the principle of devolution into more policy areas, so that only a small core of reserved powers, around matters such as foreign affairs and defence, remains.

But the two terms do not mean the same thing at all. The difference between federalism and devolution (“Plus”, “Max”, or otherwise) is not a matter of how much power is decentralised, but the basis on which such decentralisation rests.

In a federal system both levels of government – the “union” or federal level and the “state” or “provincial” level – have constitutionally guaranteed and protected powers.

Each has a degree of genuine autonomy within its own allotted sphere. This means that the powers allocated to each level have to be specified in a legal document – a written constitution, the fundamentals of which cannot be unilaterally changed by either level.

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If the federal parliament could change the powers of the state or provincial legislatures at will, those legislatures would not have real autonomy, but merely a dependent latitude to act subject to the by-your-leave of the federal level.

In Canada, for example, most constitutional amendments must be approved not only by the federal parliament, but also by the provincial legislatures of at least seven provinces, containing between them at least 50% of the population (the so-called 7/50 formula).

This division of powers between different levels of government in a federal system also has to be policed by an independent body, with authority to uphold the federal bargain and to make sure that neither the federal nor the state/provincial authorities encroach upon the other. This task usually falls to a Supreme Court.

Many federations also include a mechanism for co-operation or informally resolving differences over matters such as funding or shared infrastructure projects between the levels of government.

This may take the form of a second chamber in which the states or provinces are represented – the German Bundesrat (“Federal Council”) being the archetypical example of that type.

None of these things truly apply in the case of British devolution. Instead, the logic of devolution is that of delegation from a sovereign to a subordinate.

As Tony Blair famously remarked, “sovereignty remains in me as an English MP”, while the Scottish Parliament would be in the same position as an English parish council. The Scottish Parliament is a creature of Westminster – and what Westminster as a sovereign parliament can create, Westminster can at a stroke destroy.

Boris Johnson seems intent on that destruction. As always in the British constitution, the devolution settlement was made workable by unwritten understandings.

THE Sewel Con-vention stated that the Westminster Parliament would not normally legislate on a devolved matter without the consent of the Scottish Parliament. The Scotland Act 2016 declares the principle of the Sewel Convention, but as always in the British constitution, no one defined what “normally” meant or found a way to make it enforceable. The rule holds – except when it doesn’t.

The Scottish Government says there are 44 matters on which the UK Government’s Withdrawal Bill requires the legislative consent of the Scottish Parliament.

At PMQs this week Johnson stated, with absolute clarity, that he intends to ignore the Scottish Parliament.

Yet again, he has simply bulldozed the constitution. When constitutional protections can be abolished by the simple declaration of a prime minister that he is going to ignore the conventions, there are in effect no constitutional protections. Everything hangs on the say-so of Westminster.

Federalism – real federalism – would solve that problem.

However, there is no federal system in the world that would stop Brexit, or Trident, or would have stopped the Iraq War. This is why federalism is ultimately a dead end. If some of the main drivers of Scottish dissatisfaction with the British state concern foreign and defence policy, then the only adequate solution is one that allows those policies to be determined in Scotland, in ways that are accountable to the people of Scotland. That involves the I-word.