Is a general theory of leadership possible? Not if we focus on what it means to occupy an executive role or even to be in charge of a group. Why? Because other forms of leadership are left out, such as market leadership and being in the lead in sports. Even leading by example doesn't fit the conventional model.
Leadership is conventionally conceived along the lines described by Kouzes & Posner as a journey where the leader takes people from A to B. Leading by example might change the way people behave but without actually taking them on a journey, explicitly directing their efforts or monitoring their actions.
Also, the market leadership shown by companies like Apple and the leadership shown by league leading sports teams are also instances of leading by example. This is also true of the leadership impact that Jack Welch had on companies around the world when he set an example by initiating such ideas as the value of being number one or two in a market.
Leading by example can be defined simply as showing the way for others. Leadership in general does the same but also includes the explicit advocacy of a better way. This then takes in what Martin Luther King was doing when he spoke out against discrimination against African Americans. A chief executive promoting a new vision is also advocating a better way.
So what, you ask? Well, the advantage of broadening out our definition of leadership in this way is that the leading by example cases are clearly distinct from anything to do with management. They do not get things done through people. They simply show the way for others and nothing else. This angle gives us the key to finally and clearly differentiate leadership from management.
More importantly, if leadership is defined simply as showing the way for others, we then have a way of explaining how leadership can be shown sideways or upwards. This is crucial because we need to better engage front line knowledge workers and stimulate faster innovation. Making them see how they can show leadership without taking charge of a team, even informally, is a potentially very empowering and motivating move to make.