Two or three weeks back, some cricket scorer with too much time on his hands 'found' the missing four runs that would take the late Sir Donald Bradman's batting average from 99.96 to an even 100.
I mention this because in almost no other field is it quite so easy to work out who the greatest practictioner of it ever is. The Don's nearest rival over a reasonable number of games is fellow Australian Mike Hussey, whose average is 70 odd and falling. Of those with completed careers, the phenomenal George Headly has an average somewhere in the 60s, while most of the other greats are stuck somewhere in the 50s. Bradman was, in short, two-thirds again as good as the next best ever, and twice as good as the 'average' great player.
You don't get this certainty in most other sports, and you definitely don't get it in writing. What would you do? Tot up total book sales? That only shows popularity. Ultimately, all you're left with is a personal judgement of quality, and that is so variable that it means very little. If I want to be silly and claim that Roald Dahl was the greatest writer ever, there's absolutely nothing you can do about it. Not that I want to. Although the BFG was rather good...
But this leads to the obvious question: how do you know if what you're producing is any good? There's no absolute way of checking, so you're left with your own opinion (which in my case swings between 'It's a work of genius!' and 'It's the worst thing anyone's ever written!' often over the course of about five minutes) The opinions of assorted friends and relatives (who often try to be kind) or, if you're lucky and brave enough, the opinions of your writing group. Ultimately though, there's no way of knowing for sure, so you might as well just post it out and see.