do a backward pass of errors where you can propagate these errors through because you know it's kind of invertible the function doesn't have to be invertible but that the gradients will flow backwards if you know how the forward pass was computed so first of all they go into a discussion of back prop in the brain how can we even expect that and one cool piece of evidence is where I find is that they cite several examples where they use artificial neural networks to learn the same tasks as humans right and or as as animal