«بعض»
Some members.
«واحد من»
One of the members.
These can mark a particular affirmative by asserting a predicate of part of a class.
“Some birds are aquatic” affirms aquatic character of some birds.
“One body is inanimate” affirms the predicate of one body.
To establish a single instance is to establish at least one, which falls within particular affirmation in this traditional classification.