However, practicing Occlumency is not simply a straightforward battle of offense and defense; it requires first building defenses within the mind. This is magic that relies on intuition and willpower, and those lacking firm willpower are unable to construct defenses in their minds.
Building defenses is the first stage; the second stage is concealing memories—hiding the memories one does not wish to reveal and showing only what one wants others to see. Therefore, one must at least have a general understanding of one's own memories.
The third stage involves using existing memories to construct a factually nonexistent memory within one's own mind. The difficulty here lies in making the constructed memory reasonable and true, and since building virtual memories can lead to confusion, the practitioner needs firm willpower to hold fast to the self and not get lost in the virtual memories.
Chapter 58