Light Path Node and Mix Shader I am watching this tutorial on Light Path Nodes and in the image below the Light Path node is connected to a Mix Shader. 



When connecting the "Camera Ray" socket to the Mix Shader the lower Shader is selected as being the Camera Ray. Why is this ? Why is not the other Shader selected ?
The factor of the Mix Shader node is the fraction of the second shader that contributes to the output shader.  If the factor is 0.8 then the output shader will be 80% the second input and 20% the first input.

The Is Camera Ray output (along with most of the other Light Path outputs) is a boolean (true = 1, false = 0).  So if the ray is a camera ray (true), the value going to the mix factor will be 1 and the output will be 100% the second shader input.