"Reason" implies the need of justification: a father asks for the reason for his son's disobedience, a person gives the reason for his preference, one offers reasons in explanation or defense. "Cause" refers to an agent that produces or forces an effect or result: water and soil pollution are the root causes of mortality in the tropics (Heiser). "Purpose" denotes determination or resolution to achieve by doing something: have a purpose in life.
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