A player asked me what exactly is an outside agency.
He asked this because we were discussing when you should/should not replace your ball in the event it starts moving on the green.

I answered, "This begs the question; Were you addressing the ball when it started moving?

He said, "Lets say that I had addressed the ball and grounded my club."
I said, "Then you are attributed with the movement of the ball, case closed. Replace your ball, add one penalty stroke and finish the hole."

Then he said, "What if I had not grounded my putter?"
I asked, "Did it move by itself without help from anything else?" "Yes."

I said, "Nor the wind or rain are outside agencies, and the ball should not be replaced even if it rolls all the way to the bottom of the cup."

I'll tell you what it isn't.
In medal play;
It isn't you or your club.
It isn't your opponent or their equipment
It isn't the wind.
It isn't water.

It is an observer, a referee, a dog, bird, etc..,