In Introduction to Mechanics by Kleppner, the section on Newton's third law says that the third law is not only a physical principle but is also a logical necessity for the first two laws to make sense. I don't quite get this. These statement precedes an experiment regarding an object in a state of constant velocity of zero. Suppose the suddenly moves, the book asks what prevents us from considering that the object is not completely isolated, i.e. we always suppose that there is a force that moves and object. The book then proceeds that if this is true that the object is completely isolated then the second law would be completely meaningless. So there must be a third law which says that there is always an unequal and opposite reaction to what moved the object. How is this so?