Memory is often manipulated in the world of The Lightning Thief, and it is often redefined as well. Memory is not so much what a person recalls of the past, but mostly refers to the collective memory of a culture. The gods have been around for thousands of years, and so their memories and that culture's memory extend far back in time. This leads to all kinds of misinterpretations and manipulation of the past. It becomes clear that memory is a powerful thing, and if one could control a people's memory, one would have a lot of power over them.