Here's why:
* Abstract nouns refer to concepts, ideas, qualities, or states of being. "Method" represents a way of doing something, which is a concept.
* You cannot physically touch or see a method. It's an idea or a process.
Let's contrast it with a concrete noun:
* Concrete nouns refer to physical things you can touch or see. For example, "book," "tree," "chair."
Since "method" cannot be touched or seen, it is an abstract noun.