I worked as an intern for an archive digitizing old letters among other things. Many aspects of key historical figures are available — letters, etc. You know what they were thinking because you can read some of their personal thoughts.

In this day and age, if you’re lucky emails exist, but due to discovery, etc many entities just purge everything. Bad news, this era will be a black hole for future generations to understand. Good news is that the idiocy of this era will be toned down.