Why don't we have two nulls?

Question

I\'ve often wondered why languages with a null representing \"no value\" don\'t differentiate between the passive \"I don\'t know what the value is\"

Answer 1

I think having one NULL is a lower-common denominator to deal with the basic pattern

if thing is not NULL
  work with it
else
  do something else

In the "do something else" part, there are a wide range of possibilities from "okay, forget it" to trying to get "thing" somewhere else. If you don't simply ignore something that's NULL, you probably need to know why "thing" was NULL. Having multiple types of NULL, would help you answering that question, but the possible answers are numerous as hinted in the other answers here. The missing thing could be simply a bug, it could be an error when trying to get it, it may not be available right now, and so on. To decide which cases apply to your code -- which means you have to handle them -- it domain specific. So it's better to use an application defined mechanism to encode these reasons instead of finding a language feature that tries to deal with all of them.