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