The 5 Branches of Philosophy
Reality Is Absolute
Sum Of Parts
Metaphysical vs. Man-Made
The concepts of existence and identity apply even to thoughts, ideas, memories, etc. There is a difference between these entities and the physical objects around us. They don't exist in a physical form. Or more specifically, they don't exist as actual objects.
Mental entities are products of chemical reactions within our brains. This is the level of abstraction that they physically exist. However, we perceive them in an entirely different way. We perceive them as visual representations. We perceive them as physical sensations. We perceive them in a variety of ways. The important thing is that their physical existence is different from their content. A mental image of a triangle doesn't exist as a triangle in one's head, for instance.
Mental entities do exist, though, and so they have identity. Since our minds have the ability to manipulate these mental entities, their identities may change over time. This is not anything new, though. All objects change over time. Their identities change over time. This change, though, conforms to their identity. The identity of an object specifies how it will change.