First of all, let's start by specifying that this article is only addressed to those who have already completed Death Stranding. Well, now that you're in chapter 15, go to Peter Englert's house west of the Lake Relay City. In case you don't remember, this is the shelter where you delivered a few pizzas during your game.
Upon arriving on-site, you may find that the door of the shelter is open and you can enter. In the central room, you will then discover that Peter was none other than Higgs: this guy had fun driving you crazy during the whole game with his pizza deliveries! There you can pick up a memory card and unlock the Fount of Knowledge trophy. By recording the data on this card when you return to a relay center, you'll unlock the last parts of Higgs' diary, revealing his origins.
