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

What's actually happening to a neural network as it learns?
Next video: https://youtu.be/tIeHLnjs5U8
Brought to you by you: http://3b1b.co/nn3-thanks
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.