Here's why:
* It represents a concept: Sickness isn't a physical object you can touch or see. It's a state of being unwell, a general idea or condition.
* It's not tangible: You can't hold or measure sickness in the way you can hold a book or measure a table.
* It's often associated with adjectives: We talk about "feeling sick" or "being sick", where "sick" is an adjective describing the state of being unwell.
However, there can be some ambiguity:
* Specific illnesses: When referring to a specific illness, such as "the flu" or "chickenpox," these become more concrete, though still technically abstract.
* Metaphorical usage: Sometimes, "sickness" can be used metaphorically to describe something bad or harmful, such as "sickness in the economy."
Overall, while "sickness" can sometimes blur the lines, it's primarily an abstract noun due to its intangible and conceptual nature.