Not some slogan, tag line, or inherited mission given to you by someone you don't even know, but rather your true vision. What you see as the reason you are here. What you hope, plan, and expect to accomplish in your lifetime.
That's a big one, isn't it? Big because it requires thought and effort and big because as we form our vision we may have absolutely no idea how to accomplish it. If it's too easy it's not a vision, it's just an incremental goal. A vision is tough work.
Why go at it alone? Why not get the help you'll need to bring that vision about? Why not find out if that vision even makes any sense (the world probably already has enough Don Quioxotes tilting at windmills out there).
A vision is only as powerful as those who share it.
Build your vision with your allies. Develop your vision with your friends. Create your vision with your mentors. Get the help you need.
You'll need it.
-- Doug Smith
Front Range Leadership: Training Supervisors for Success
doug smith training: how to achieve your goals