title: Why must a nonlinear activation function be used in a backpropagation neural network?
body: <p>I've been reading some things on neural networks and I understand the general principle of a single layer neural network. I understand the need for aditional layers, but why are nonlinear activation functions used?</p>
  
  <p>This question is followed by this one: <a href="https://stackoverflow.com/questions/9785754/what-is-a-derivative-of-the-activation-function-used-for-in-backpropagation">What is a derivative of the activation function used for in backpropagation?</a></p>