This achievement is earned at the junction at the end of the third act where you play Paul again. During this junction you need to decide whether you want to trust Martin Hatch of Sofia Amaral. This achievement is earned for choosing to trust Martin.
Note that you can make this choice, then immediately replay this section and make the other to unlock both achievements if you don't want to worry about replaying it toward the end of the game. Make sure you do not replay a junction choice until you have completed all of the collectible achievements since doing so will change/move many of the collectibles, essentially requiring you to refind them.