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When he was very young, one of my sons, having watched a tractor pulling a plough in a neighbouring field, opined that the purpose of the operation was to turn up worms for the seagulls that were squabbling over the newly turned furrows. I thought then, and I still think, that this explains 99 per cent of the observable facts about ploughing while still being totally wrong. But in this it is not alone. Sooner or later all our observations have to be filtered through insider knowledge.