Consider, for a moment, that you would like to introduce bonus action points based upon a mental ability score to your game. Don't worry about the balance implications just yet, as there are other houserules in play which make that rather complicated to explain. From a purely thematic or aesthetic perspective, which ability score do you think best fits the idea of gaining extra action points - much in the same way as Dexterity grants a bonus to initiative, or Constitution to hit points. Would it be Intelligence, Wisdom or Charisma? Why do you think that mental ability score is the best fit?