of evidence against there being something like back propagation in the brain so the question is how do neural networks in the brain learn and they they say there there can be many different ways that neural networks learn and they list them up in in this kind of diagram where you have a network and it maps from input to output by having these weighted connections between neurons so the input is two-dimensional and then it maps using these weights to a three-dimensional hidden layer and usually there is a nonlinear function