doubt vs purpose

doubt

verb
  • To be undecided about; to lack confidence in; to disbelieve, to question. 

noun
  • Disbelief or uncertainty (about something); (countable) a particular instance of such disbelief or uncertainty. 

purpose

verb
  • To have (an) intention, purpose, or design; to intend; to mean. 

  • To have or set as one's purpose or aim; resolve to accomplish; intend; plan. 

noun
  • meaning for existing or doing something. 

  • Resolution; determination. 

  • Function, role. 

  • The end for which something is done, is made or exists. 

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