The story The Three Questions by Leo Tolstoy tells the tale of a king who strives to find answers to what he considers life's most important, yet hardest to answer, questions: 

• What was the right time for every action?
• Who were the most necessary people?
• What was the most important thing to do?

Through searching for the answers, the king stumbles into a situation with a subject of his kingdom that he wronged in some way but then ended up helping survive a fatal wound.  This subject was on his way to kill the king for his crimes and through being healed by the king, found his way back into the king's good graces.  The questions for the king get answered through his experience with his subject.  The right time is the present time.  The necessary people are who is with you in the present.  The most important thing to do is to do right by the person you are with.  How does this connect to the question of purpose?  In the answer to the third question from the story is the answer to the former.  Purpose is determined by what is happening in the here and now.  Whatever we are doing that is doing right or good by the people we are with is what shapes our purpose.