These are a few notes on what is known as Postel’s Law or the robustness principle. The law goes as follows:

Be lenient in what you accept and be strict in what you put out


  • The robustness principle is known as Postel’s law after Jon Postel wrote it in an early specification of the internet protocol in 1980
  • The principle is a design guideline for software was originally “Be conservative in what you send, be liberal in what you accept
  • But it can be applied widely areas as Garry Tan says:

I came across Postel’s Law in the below video by Garry Tan.

According to Garry:

Being liberal with what we accept goes hand in hand with empathy for what the sender was trying to say. If you can give a more charitable read to the things that you take in, then it adds slack to the system.

I hadn’t heard about Jon Postel and his legacy as the ‘God of the Internet’ previously. If you’re interested, here is a picture of him pointing at a map of the internet back in the days 👇

If you are interested in learning more about Jon Postel, this article by the Internet Society is a good place to start: A ten year tribute to Jon Postel: An Internet visionary.