idea vs raison d'être

idea

noun
  • A purposeful aim or goal; intent 

  • An abstract archetype of a given thing, compared to which real-life examples are seen as imperfect approximations; pure essence, as opposed to actual examples. 

  • A conception in the mind of something to be done; a plan for doing something, an intention. 

  • A musical theme or melodic subject. 

  • More generally, any result of mental activity; a thought, a notion; a way of thinking. 

  • An image of an object that is formed in the mind or recalled by the memory. 

  • A vague or fanciful notion; a feeling or hunch; an impression. 

raison d'être

noun
  • Reason for being. The claimed reason for the existence of something or someone; the sole or ultimate purpose of something or someone. 

How often have the words idea and raison d'être occurred in a corpus of books? (source: Google Ngram Viewer )