There's a wonderful single-frame Gary Larson cartoon with a caption that reads something like 'Edgar finally finds his purpose' and shows a young man pulling a bizarrely unidentifiable object from behind a sofa.

I've been thinking about purposes tonight. Any self-help manual or motivational literature will tell you that to be truly successful you need to find your purpose. Fair enough, but what if you have absolutely no idea what it is?

I watched the excellent biopic of Ghandi with Ben Kingsley today. Now there was a guy that knew his purpose. Martin Luther King - there's another one. Nelson Mandella, Mother Theresa and even despots, dictators and tyrants like Stalin and Hitler all lived with a grand vision of what they wanted to achieve and how they were going to do it.

In the face of such lofty ambitions, I find it hard to come up with what my purpose is. I'm not going to be a great leader or the instigator of social change, nor is it likely that I will seize ultimate power in a country and put anyone that doesn't agree with me to death.

To be honest, I don't even know that I have a purpose. The problem is that, although I am passionate about lots of things, I don't care about any one thing enough to let it dominate my life. There isn't one thing that I love to do above all else that I wish to shape my life around.

I guess that most people are like that. When we look at people we consider to be hugely successful, they have often pursued one thing relentlessly to the exclusion of all else. I don't think I'm prepared to do that - I'm really not that bothered about any one thing.

I suppose what has set the old cogs turning in my brain tonight is the slow dawning of the fact that I'm unlikely to do anything with my life that will get me a place in the history books. That thought is kind of depressing.

Now I know that might sound a bit insane, but bear with me for a minute while I explain. Like many of us, I feel that I've been kind of sitting round waiting for my life to begin. I still feel like I did as a teenager, that I don't know what I want to do and haven't made any decisions about my future. The plus side of thinking like that is that it means that, in theory at least, literally anything is possible.

When you're young that's pretty much true but as you get a little older doors get closed to you just by time. I know at 31, I'm hardly ready for the scrap heap just yet, but I realised the other day that I'm now really too old to make a career playing sport professionally. This means that I'm never going to score that hat-trick in the world cup final or win an olympic gold medal. I'm just too old. I didn't close the doors to those things but time closed them for me.

Similarly, I'm coming to the conclusion that if I were going to find my big purpose and achieve things that my great-great grandkids will learn about in school, I suspect I would have found it by now.

It was a bit of a shock to realise that my little life isn't really going to be that important in the grand scheme of things. I guess I'll just trundle along, try and have a nice time and one day I'll die and, friends and family apart, the rest of the world will carry on without blinking an eye. I don't doubt at all that it's a ridiculous thing to complain about, but I was honestly a little shocked when I had that thought.

And then it occurred to me that you don't have to change the world to live a significant life. Our lives touch other lives every single day. Somebody like Ghandi touched millions of lives but that doesn't devalue us because we live on a smaller scale.

There are people in this world that love me. There are people in this world who have been positively affected by my life. They won't teach it in tomorrows' schools but that doesn't matter - it is enough to know that it's true.

I don't know if I will ever find my purpose - or if I even have one; but for now I can take comfort in the thought that even if I don't come up with it, it won't all have been a complete waste of time!