proverb vs unknown quantity

proverb

noun
  • A striking or paradoxical assertion; an obscure saying; an enigma; a parable. 

  • A drama exemplifying a proverb. 

  • A phrase expressing a basic truth which may be applied to common situations. 

  • A familiar illustration; a subject of contemptuous reference. 

verb
  • To name in, or as, a proverb. 

  • To write or utter proverbs. 

  • To provide with a proverb. 

unknown quantity

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

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

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