Most people would count themselves as rationalists, yet nearly everybody indulges in a sort of unthinking casual superstition. For example, how many of us have had this thought: "Oh typical, it's raining! Every time I go out without my umbrella it bloody well starts to rain." If we examine this thought, we would see it contains the most extraordinary mystical egotism.

For there really to be a connection between our leaving our house without an umbrella and it raining we must first of all presuppose some sort of all-powerful godlike entity. One of the tasks this god-like entity will have assigned to itself (along with maintaining all the planets in the Universe in their orbits and keeping an eye on entropy and stuff) will be staking out your gaff to see when you pop out without your Knipps Red- dot automatic folding brolly. Once this all-powerful being sees you are sans parapluie it will then cause minute drops of water in clouds to coalesce to form larger drops, and when these drops become sufficiently heavy they will fall as rain wherever you happen to be. It doesn't seem likely, does it, when you really think about it?

But it's really difficult to break the hold of these superstitions. One of the beliefs that I continue to hold on to is that everybody, once in their life, gets what I call their "Big Chance" - an opportunity, a big break which, if seen for what it is and seized with gusto, will be the defining moment in a person's life, providing them with happiness and success for the rest of their days.

Over the years millions of people must have noticed how cats' eyes shine in the dark, but only one - a Yorkshireman - thought of adapting the principle for use on roads, making himself a millionaire in the process. If you or I had come into our laboratory in 1928 and noticed that a mould, Penicillium notatum, contaminating a bacterial culture had destroyed the bacteria in its vicinity, we would probably have thrown the culture away and set up the experiment again. Luckily, Alexander Fleming had the nous to look further into it.

Now, until recently, I've thought that my Big Chance was my wife spotting an advert in Private Eye for auditions for the about-to-open Comedy Store in l979. I went along and they offered me the job of MC. From that, my entire career in TV - and ultimately this column - sprang.

But of late I've begun to think that maybe that wasn't my "Big Chance". After all, I mean it's all right being me, but it ain't that great. If I'd recognised and seized my "Big Chance" then surely I should be having such a good time that I spend my days laughing like a drain and drinking Irn-Bru out of ballerinas' Air Jordans. Maybe getting the job at the Comedy Store was just a Little Chance, just a run-of the-mill life event and just maybe I haven't had my "Big Chance" yet.

Or maybe, big horror, I actually had my Big Chance and I didn't notice it or take it. This thought has got me so worried that I find myself constantly searching my memory for missed opportunities. Did somebody take me to the top of a high mountain in 1979 and show me the world and all its wealth and offer it all to me and did I refuse, saying it was very nice but I had to go to a Graham Parker and the Rumour concert?

Or I try to find a dim recollection of a Broadway producer rushing out of a theatre and offering me the lead in a giant tap dance spectacular alongside Ruby Keeler. But, oddly, I can't seem to recall anything of that nature.

Maybe it might not have been anything that obvious. Maybe my Big Chance was as simple as somebody in a pub saying, "Would you like a peanut?", and if I'd said yes rather than no I might have got chatting to that person and they might have turned out to be the inventor of a rocket-powered anti-gravity motorbike and I would have become friendly with them and they would have let me be the sole user of their invention. Which would be housed in a cave in the Arizona desert and I would have become Rocket- Powered Anti-Gravity Motorbike Man and I would go around the world solving crimes and righting wrongs with my rocket-powered anti-gravity motorbike and they'd build statues of me and my cat Tiger, who'd be renamed "Rocket- Powered Anti-Gravity Motorbike Cat", in the main squares of every town in Latin America.

However, I refused the peanut the man offered me in the pub because, frankly, he was odd looking (which I suppose an inventor living in a cave in the American South-west would be). So it didn't happen.