Ask HN: Why can't we write down something in our head?

It is built in. But since we learned writing and reading, we're underutilizing it, so it's decaying.

Plato, putting words in the mouth of Socrates in Phaedrus wrote:

...for this discovery of yours [writing] will create forgetfulness in the learners’ souls, because they will not use their memories; they will trust to the external written characters and not remember of themselves.

http://en.wikisource.org/wiki/Phaedrus#On_the_decline_of_Gre....

Chess players can play without the chess board.

Some writers prisonners wrote books in their head.

Similarly, if you were separated from your computer you could still write programs, in your head. You would have to be very carefull, but you could reach a stage where the program is debugged well enough that you could just type it out (once you've left out the the prison), and have it run on the first time.

Just avoid TV and its brainwashing (cf. Fahrenheit 451, http://www.laweekly.com/news/ray-bradbury-fahrenheit-451-mis... ).

It's a function humans can develop, and in the past did. Once writing was common, it was less necessary, and the effort to develop it more often went elsewhere, though people still memorize short things frequently, and longer things occasionally. (Interestingly, many find that the act of writing notes aid memory even when the notes are not later consulted.)

How short are we talking about? Pretty sure most people can remember a sentence? I very rarely make notes tbh. The "don't forget to think of it" part is a bit trickier IMO. You have to have some kind of a trigger. With an odd, rare thought it could be tough without a non-mental trigger.