paradox vs unknown quantity

paradox

noun
  • A person or thing having contradictory properties. 

  • A claim that two apparently contradictory ideas are true. 

  • An unanswerable question or difficult puzzle, particularly one which leads to a deeper truth. 

  • The use of counterintuitive or contradictory statements (paradoxes) in speech or writing. 

  • An apparently self-contradictory statement, which can only be true if it is false, and vice versa. 

  • A state in which one is logically compelled to contradict oneself. 

  • A counterintuitive conclusion or outcome. 

  • A thing involving contradictory yet interrelated elements that exist simultaneously and persist over time. 

  • The practice of giving instructions that are opposed to the therapist's actual intent, with the intention that the client will disobey or be unable to obey. 

unknown quantity

noun
  • A person or thing whose nature or value is a mystery. 

  • Used other than figuratively or idiomatically: see unknown, quantity. 

How often have the words paradox and unknown quantity occurred in a corpus of books? (source: Google Ngram Viewer )