What is backpropagation really doing? | Chapter 3, Deep learning

What's actually happening to a neural network as it learns?
Help fund future projects: https://www.patreon.com/3blue1brown
An equally valuable form of support is to simply share some of the videos.
Special thanks to these supporters: http://3b1b.co/nn3-thanks
Written/interactive form of this series: https://www.3blue1brown.com/topics/neural-networks

And by CrowdFlower: http://3b1b.co/crowdflower
Home page: https://www.3blue1brown.com/

The following video is sort of an appendix to this one. The main goal with the follow-on video is to show the connection between the visual walkthrough here, and the representation of these "nudges" in terms of partial derivatives that you will find when reading about backpropagation in other resources, like Michael Nielsen's book or Chis Olah's blog.

Video timeline:
0:00 - Introduction
0:23 - Recap
3:07 - Intuitive walkthrough example
9:33 - Stochastic gradient descent
12:28 - Final words