Everything needs everything else. Doesn't it?

Mekhtoub, they say. 'It is written.' Well. When I say 'they', I mean King Mob in the second volume of Grant Morrison's The Invisibles , when he kisses Ragged Robin .

It is written - isn't it weird how those words unspool so easily in the mind? How quickly the contingent, the chaotic, becomes the way it's always been? How quickly the facts on the ground are established?

Golo Mann criticised AJP Taylor's idea of the inevitability of the second world war for committing this fallacy. The Second World War needn't have happened. The Third Reich needn't have happened. Hitler needn't have happened. Things could have been totally different. This is what history is: the record of all the things that need never have happened.

But isn't it odd how things, in retrospect, take on the mask of inevitability? Take the Williams Sisters : playing each other in yet another Wimbledon final. Doesn't that, these days, seem somehow inevitable? Yet it needn't have been. You can trace the chain of causality all the way back from this particular tournament to their father's decision to train them as tennis pros. At any number of points on the way, things could have turned out differently.

There is a fossilised organism in the Burgess Shale called Pikaia . It may - there is some doubt - be the first chordate, or vertebrate, creature in the fossil record - our earliest ancestor. And it, too, need never have existed. Stephen Jay Gould believes that, if Pikaia had not survived, if it had not been an evolutionary relative success, then none of us would be here. Never mind Hitler, the Williams Sisters or whether you should have danced with that girl at the prom: if something had gone wrong during Pikaia's time, you wouldn't be around to regret these things.

Yet we act as if all this was inevitable. And the concomitant result of this is that we assume that the things we take for granted in our lives are inevitable, too. Our house, our job, our marriage, our physical health, our overdraft limit, our sanity. Sadly, it ain't necessarily so. Everything we have can change in an instant. In the moment when a decision is made, when a thing is said, we can find ourselves living in worlds of which we never even dreamed. Nothing is inevitable: everything is contingent.

It's a lesson we need to learn again and again and again, and I'm getting a painful refresher course now. Things which, for a long time, I took for granted are changing in ways I could never have anticipated. What was written is being erased: a new hand writes now, and while it may not exactly scrawl Mene, Mene, Tekel Upharsin it's a bit too close for comfort. The die is cast, but hasn't landed yet: and, if anyone feels like praying for me, now may well be the time.