e. a notion formed in the mind, but here it is implied to apply to relatively wide and important things, such as ideas about how society should be organised, or ideas about how life began, or stuff like that, rather than trivial or day-to-day ideas, such as "I had an idea about how to fix that dripping tap".
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It is fundamentally the normal meaning, i.e. a notion formed in the mind, but here it is implied to apply to relatively wide and important things, such as ideas about how society should be organised, or ideas about how life began, or stuff like that, rather than trivial or day-to-day ideas, such as "I had an idea about how to fix that dripping tap".