To live you have to move. In most places, that movement is taken for granted. Moving across the city to work, moving from the office to a café or a pub afterwards, moving our children from our homes to a park, moving around at night, moving by car, moving by foot, moving any way we like.
But in a place like Baghdad, it's not so simple. Moving is stop-start. Though movement is everywhere you look, it is fitful. It is never free. It is always watched. And it is always, unavoidably, interrupted.
The checkpoints that do that are numerous. Nobody will say exactly how many - perhaps nobody really knows - but there are hundreds that are permanent, made of steel and concrete and iron. And there are hundreds more that are mobile, made of the same. When security is tight, which it usually is, people here guess there may be 1,000 dotted across the city: its violence valves.
As they open and close, controlling the flow of life through this town, they give everything an unusual rhythm. At a shallow glance, Baghdad is a city that teems in the way that only the world's busiest do. People appear from everywhere, going everywhere. Traffic policemen stand in the middle of humming intersections, cars inching along, hawkers selling nuts to passengers through the windows.
But there are jolting sounds. The sirens and souped-up horns of a military convoy coming through the traffic, forcing cars to the side of the road. Some of the armoured vehicles have turrets that masked soldiers stand up in, training machine guns on disinterested commuters, all too used to that.
Up ahead, the checkpoint that's causing the traffic to stagnate is manned by a large squad of watchful troops. Some cars are waved through immediately, some are quickly checked, and some are asked to drive into a waiting area for a more thorough search. There are pedestrians, too. Some pass through easily, some produce identification papers, some are spoken to for a longer time.
A group of men start arguing, shouting at the men with guns, demanding to know why they’ve been stopped. Diffident, a young mother holds her small boy's hand and waits in silence, looking at the ground.
A huge English-language advertisement hoarding for a foreign electronics company, its global promotional campaign not altered at all for Iraq's market, looms down over this scene.
"Life is good," it says.
But what the checkpoints say is that, here, life is delicate.
Sullied but salvageable
Baghdad is a city plagued by bombers. The world’s media has long since departed, but they remain. With growing regularity, they target civilians - ordinary people - and they show no mercy. Car bombs are parked outside ice-cream parlours, men walk into cafes and blow themselves up.
The fact that anybody is on the streets at all is remarkable - heroic even.
"In Baghdad, you can die any time," one man told me. "So there's no point being afraid."
It's that fact, the random nature of the attacks, that is perhaps most terrifying of all. Often, they happen when people are trying to live normally, to behave as they did in the days before war.
A couple of weeks ago, a former military commander, with tears in his eyes and shock in his body language, told me he had noticed a suicide bomber in the seconds before he detonated himself. It was when he stopped, quiet and calm beside some people drinking tea on street-side tables, that the old army man knew. By then it was too late. Within seconds, 10 people were dead in the road.
The city that Baghdad could be - and once was - is ever present, suggesting itself somewhere underneath, as if today, with its soldiers and watchtowers, was just sketched on top in pencil, a great painting still there, sullied but salvageable if only it could find a suitable restorer.
Yet everyone here knows that, perhaps in a restaurant or waiting at a checkpoint, they could someday hear that bang, often more of a pop, that means lives ended and others forever altered. Some who die will make eye contact with the bomber just before detonation and, instinctively, realise what is about to happen. What must both people think during that last brief moment of human interaction?
Everyone also knows there's little you can do to protect yourself from such things. But tomorrow - with a mixture of bravery, defiance and hope – they will again move around the streets of their town. And live.
Follow Barry Malone on Twitter: @malonebarry