Here's why:
* Proper nouns are specific names of people, places, or things. They are always capitalized. Examples: John, London, Amazon River.
* Common nouns are general names for people, places, or things. They are not capitalized unless they are at the beginning of a sentence. Examples: person, city, river.
"Slave" and "act" are common nouns because they are general terms.
* Slave refers to a person who is forced to work without pay and is not allowed to leave.
* Act can refer to a deed, a performance, or a part of a play.
Let me know if you have any other questions about proper nouns!