When Martin McGuinness shakes hands with the Queen today, at Belfast's Lyric Theatre, we'd do well to remember the symbol behind the symbol. This most ubiquitous of human gestures shows two things: that you're not armed and that you're prepared to embrace the other party.
That's why McGuinness and Her Majesty shaking hands is such a big deal. It's the acknowledgement that both exist and that they are prepared to accept the situation the way it is.
Following last year's historic, diplomatically-adept visit by the Queen to the Republic, today's event will not be so universally ground-breaking. But it will nonetheless be momentous.
It shows that Sinn Fein is listening not just to its own voters - the vast majority want the handshake to happen - but to the people of Northern Ireland as a whole.
It also shows that, even for Sinn Fein, ideological 'purity' is much less important than humane transactions.
Welcome to the real world. Most of us have for years had to settle for the reality of acknowledging the existence of 'the other', for just getting on with it.
No matter which side of the fence you come from, you're surrounded by the other. Most nationalists didn't have nosebleeds during the recent Jubilee celebrations, most unionists won't go into meltdown when the Pope eventually comes here.
After 20 years of 'peace' and deals and petty standoffs, arch ideological objections begin to look as incomprehensible as flared trousers or a liking for Lenny Henry.
That was why, though Sinn Fein's reluctance to shake hands with HM was irritating, it had to be endured.
It seems a given now that SF must go through a 'dark night of the soul' before each historic act of what turns out to be leadership. They struggle manfully with what appears obvious, as if there were some nuance of meaning in each act which eludes the rest of us and which only the theologians of the Ard Comhairle can clarify.
And then, as has happened repeatedly, the verdict is ... The Bleedin' Obvious.
The rest of us have learned to endure this hand-wringing and in some way to understand it. We've learned to endure the endless talk of 'big asks'. Everything is a 'big ask' for SF. As if it's not an ordeal for many unionists to watch Martin on TV as Deputy First Minister. As if, as if, as if ...
We endure it because we now know, in spite of the odd renegade, Sinn Fein as a party will always do the right, the sensible, the obvious thing. For most of us here in the real world, every single day involves countless 'asks' of various sizes - big, small, enormous, so-so. We could all find problems with our wee peace - none more so than the Queen who lost family members due to the IRA.
This isn't to belittle the handshake. Far from it. Martin McGuinness deserves congratulations for having the courage to be the one to cut the Gordian knot. It would be hard to envisage SF getting this far if it had been any other 'leading republican' in the frame.
Uniquely amongst the party leadership, McGuinness is the one who has grasped most intuitively the meaning of the Stormont Agreement beyond the sub-clauses. McGuinness knows the Agreement means doing things which make us uncomfortable and that we are then going to have to learn to be comfortable doing it.
It's his centrality to finding a critical path through every moment of crisis over the last 20 years, even the most complex and personally challenging events which required the immediate and right action, which makes McGuinness the Michael Collins of our times.
There is nothing more uncomfortable than grasping the hand of our natural enemy. But it only has meaning as a public act when both parties actually represent in their own persons, in their own lives and histories and journeys, the breadth and depth and energy of opposing forces.
The Queen does so by symbol, inheritance, grace and by her personal losses; McGuinness by his route to power, by what he has represented for decades within the Republican movement, by his credibility, by his acknowledgement of force, then by his extraordinary intelligence in treating as an equal the historic foes of his and many other generations.
Today must be a historic day when the last of the shibboleths bites the dust. It isn't quite Michael Collins and Winston Churchill and Lloyd George.
It's much greater than that.