The difference between proper nouns and specified common nouns (eg 'my car') [closed]

Proper nouns are supposed to denote a specific entity. Why aren't the phrases "my car", "her hat", "their dinner", etc., considered proper? They each refer to specific entities, but aren't usually considered proper.

The reason is that the car, the hat and the dinner in your examples are specific only when the person or people referenced by the possessive pronouns are known. Not only that but when the pronouns reference other people the phrases reference other cars, hats and dinners.

For instance if you say "That is my car" then the phrase will refer to the car owned by Newman and we can replace "my" with "Newman's. If I say "That is my car" then the phrase will refer to the car owned by Ben and we can replace "my" with "Ben's". We also replace your new Mercedes with my old Nissan:-) Similarly if I say "That is your car" I am referring to the Mercedes but if you say "That is your car" (referring to me) you are referring to the Nissan.

The previous paragraph, of course, includes four examples of true proper nouns. These are the nouns with the first letter capitalised (Newman, Ben, Mercedes and Nissan). The point about these is that, although there are many people called Newman and Ben and also many cars made by Mercedes and Nissan the people are known by the names in many contexts and the brand names always refer to the same brand. That is you can't, truthfully, point to a Nissan and say it's a Mercedes but I can point to a Nissan and say "That's my car" whereas other people can point to other cars and say the same thing.

The point is that proper nouns refer directly to specific people, places organisations and sometimes animals and other things as well and only mean other people, places organisations and so on when two or more of them have the same name (me and Ben Fogle or Boston, Massachusetts and Boston, Lincolnshire for example) but phrases including a possessive pronoun and a noun mean different things depending on the person identified by the personal pronoun.

A proper noun has a name, and the name is capitalised when referring to that thing (or person) by its name.

A common noun is not a name, and a capital letter is not used.

Thus "my car" refers to a particular car, yes, but not by its name. You might have a VW Beetle and refer to "my Herbie" because Herbie is the name [proper, capitalised] of the car [common, uncapitalised]. The same applies to "my Peter" instead of "my husband" — the wife is referring to a particular person in both cases, but common nouns aren't capitalised.

Side note: in English up to around 1750, and still in some other languages, it was indeed usual to capitalise all nouns; proper nouns were distinguished by setting the whole word in italics or as all-capitals. But that's not modern custom and usage.