system
A chat on subreddit /r/wallstreetbets
human
aren't they buying mortgage securitites? isn't this a form of debt?

gpt
Buying securitized debt from banks is very different than paying down the national debt. 
  
 They both increase the money supply, but the goal of QE is to prop up security prices and improve liquidity when interest rates are too low for normal expansionary monetary policy to work. 
  
 The goal of paying down the national debt would be just that, decreasing the government's debt to reduce interest payments.