The Backstop is not just about the border. It is not a technical matter. It is not just about what happens at 200 crossing points.
It is about the people of Northern Ireland, and giving all of them (not just a majority) the freedom to be who they are, and a sense of belonging.
But the present debate in the UK Conservative Party about replacing the backstop, seems to assume that it is all about technical fixes and invisible border posts , and that some yet to be discovered combination of IT and lasers would remove the need for physical customs posts, and that would then solve the entire problem. That is a mistake.
The backstop is about far more than this. It is a recognition of the fact that, in Northern Ireland there is a population some of whom feet they have exclusively British identity and allegiance, some of whom feel they have an exclusively Irish identity and allegiance, and some of whom combine these allegiances comfortably enough.
The backstop is a recognition of this fundamental divide, which has led to so much suffering in the past, and an attempt to sustain the arrangements that ended that suffering.
The Belfast Agreement of 1998 transcended these divisions through provisions for intense North/ South and East/West cooperation, that would allow all three groups, described above, to feel fully at home in Northern Ireland under any present, or future , constitutional arrangements.
This was easy to envisage as long as both parts of Ireland remained in the EU, because EU rules facilitated and underpinned free and easy cooperation both North/South and East/West.
In such a context, territorial “sovereignty” became less of an issue, because it was overlaid by structures of free cooperation enshrined in EU law.
Brexit changes all that in a radical way. It brings territorial sovereignty back into the centre of stage in a way that threatens the Belfast Agreement settlement in a deeply fundamental way. I believe that Theresa May came to understand this, and that that explains her acceptance of the backstop.
Most of those contending to take her place in the Conservative Party leadership do not seem to do so.
In the agreement of March 2019, the EU side has given the UK very strong assurance of its good faith in seeking to find an alternative to the backstop.
But that will only work if the UK side really understand why the backstop was put there in the first place.
I do not believe that the contenders for Conservative Party leadership have taken this on board.