and thus is a function of the output activations. Of course, this cost function also depends on the desired output yy, and you may wonder why we're not regarding the cost also as a function of yy. Remember, though, that the input training example xx is fixed, and so the output yy is also a fixed parameter. In particular, it's not something we can modify by changing the weights and biases in any way, i.e., it's not something which the neural network learns. And so it makes sense to regard CC as a function of the output activations aLaL alone, with yy merely a parameter that helps define that function.