Here's why:
* Abstract nouns refer to ideas, concepts, qualities, or states of being that cannot be touched or seen.
* Concrete nouns refer to things that can be touched, seen, or experienced with the senses.
"Notion" represents an idea, a thought, or a belief. You can't physically touch or see a notion.
Example:
* Concrete: "I bought a new computer." (You can physically touch a computer)
* Abstract: "She had a sudden notion to quit her job." (Notion is an idea, not a physical object)