Here is the full list of all 12 Path to Mnemosyne achievements.
Unlock the third flashback.
Unlock the second flashback.
Unlock the first flashback.
Open the door of your senses.
Open the door of your mind.
Unlock the fourth flashback.
Find the first hidden fragment.
Unlock the fifth flashback.
Unlock the sixth flashback.
Open the way to Mnemosyne.
Reach the end of the path without the fragments.
Reach the end of the path with the fragments.