I seem to remember seeing the idea that “all possibilities equally likely” is sort of a “default prior”. In the case of life on Mars: Imagine getting all your information about life and Mars in little dribs and drabs, each one of which lets you update your probability of life on Mars. The place you start from (before you know stuff like what DNA is and whether Mars has an atmosphere) is 0.5.
I seem to remember seeing the idea that “all possibilities equally likely” is sort of a “default prior”. In the case of life on Mars: Imagine getting all your information about life and Mars in little dribs and drabs, each one of which lets you update your probability of life on Mars. The place you start from (before you know stuff like what DNA is and whether Mars has an atmosphere) is 0.5.