There's a fantastic text guide here:
https://www.walkthroughking.com/text/talosprinciple.aspxTo get this one, you need to wait until you near the end of the game. You will have a conversation at the terminal. The choices should be:
-I accept that some of my beliefs face difficult challenges.
-I equated personhood with humanity, which was a mistake.
-There's nothing else I'm prepared to doubt quite yet.
-I am unique.
-So long as I might have chosen otherwise I can make a difference.
-Does the path being predetermined mean it isn't worth taking?
-Do you think there can be no new ideas? Nothing better?
-If there's no point in anything, why are you talking to me?
-Are you avoiding the question?
-(finish the conversation)
After you get the achievement, quit out and restore from the previous checkpoint. You then should make sure that you've followed the other two choices before continuing on.
Make sure you've done this before you go to the third:
This is the third and final conversation you should have before continuing with the story: