The Golden Mean has many names, it's all based on the "Golden Ratio" in maths.
It occurs in nature (your fingers are in the golden ratio, as is your hand/arm/elbow)
In terms of composition, it's better known as the
Golden Rectangle
and it's something that artists and architects have been using for hundreds of years.
If you look at a
Golden Spiral, look at where the spiral curles around in the lower right.....
Now compare that to the lower right point in the Rule of Thirds.....
Not a million miles away, are they?
The same would apply to a GS drawn on the other sides, you would end up with four spirals roughly at the four intersection in the RofT.
You can also see the Golden Rectangle lines there, roughly where the Rule of Thirds lines are....
It seems to me the Rule of Thirds is an approximation of the Golden Mean stuff, so when taking pictures, expand the Rule of Third lines out a touch to the corners et viola, you're at the Golden Mean points.