Today, proper nouns versus common nouns. First, a mangle:
Nouns identify people, places and things. A common noun refers to general or non-specific items: a cat, a car, an uncle. All of these may be given individual names — Garfield, Ferrari Testarossa, Uncle Tom — but they simultaneously maintain their generality: Garfield is a cat, a Testarossa is a car, Tom is an uncle.
A proper noun relates to something specific, perhaps even unique; and thus it is often a name, whether of a person, a place, a product or whatever. As the above examples show, a proper noun is distinguished by the use of an initial capital letter. Hence a person might be named Rodriguez or Zhi Peng, even if that person is also more generally a father or an uncle. Specific places are also generic types: Tasmania and Goa are, respectively, states in Australia and India. Products follow a similar pattern: Sellotape is a brand of sticky tape; Hibiki is a brand of Japanese whisky.
Sometimes the same basic noun has common and proper variants, and this trips people up. For instance, the Greek deity Zeus and his Roman counterpart Jupiter have individual names, but are also (common noun) gods. However, Christianity, being monotheistic and thus not dealing in general gods, but a single god, converts that common noun into a proper noun: the Christians’ god is called God.
This rule applies even if the writer is an atheist: this is not a religious issue, but a grammatical one. After all, Zeus still has a capital Z even though no-one believes in him any more, and since only God is named God, it would be as incorrect to refer to Zeus and Jupiter as Gods, rather than gods, as it is to use the proper noun without an initial capital.
