Here's why:
* Abstract nouns refer to ideas, concepts, qualities, or states of being. They cannot be touched or physically experienced.
* Concrete nouns refer to things that are tangible and can be perceived by the senses.
An assertion is a statement that is declared as a fact, a belief, or a claim. It is an idea or concept, not a physical object.
Let's consider some examples:
* Concrete: Table, book, dog, rain
* Abstract: Happiness, justice, freedom, assertion
Since you can't touch or see "assertion," it's an abstract noun.