if you knew in one layer and on the top layer of course you know if you knew that okay I got this from my forward pass but I would like to have this this is my desired output right so in the output layer you get this this is your error signal if you knew you you you could compute an error right here this is what you do in the output right now in back prop we would back propagate this error along the layers but now we don't do this instead of what we do is we use this G function to invert the F function right and by that what we'll