Superstition implies a belief that the physical world has meaning: something knows you chose not to carry a brolly, and decided for that reason to make it rain. The levers of the cosmos respond to the light touch of your thoughts. Idiocentric. So to deny superstition is to deny this kind of meaning, to assert one’s insignificance in the great scheme of things, to be a nihilist. Don’t worry about the brolly – it will or won’t rain regardless.