Learn English – Would “communists” be considered a proper noun

The author of the passage determines if "snake" is a proper noun, that is, if the snake's name is The Snake. If it is then "The Snake" should be capitalized. If, instead, it could be any snake, but happens to be this one, and the snake's name is actually Bob, but that isn't important enough to even note, the author may write "the snake said ...".

Note: The Snake might have multiple names, but if the author puts the capital letter on Snake then that is a signal to the reader that this snake is important, and indeed later if you see the words "The Snake" in the same work you might infer that it is the same snake.

Example:

"the architect [who designed this building] was either a certified genius or authentic wacko."

(any architect)

"I am the Architect. I created the Matrix."

(character is named the Architect)

Proper nouns should name specific people, places or things - "specific" being the key word here. In some cases specificity is immediately apparent, e.g. in the term "Singapore Airlines" (after all, there is only one airline with that name!).

In other cases, there is a grey area and I would say that fame/notoriety also plays a role in determining specificity. To use your example, one would refer to the Guantanamo letters if they don't contain any content that is particularly newsworthy. If, on the other hand, these letters happen to disclose some salient information that could result in a political scandal, then these letters would quickly garner attention and become intimately linked to this political scandal in the minds of people. Such a link would imbue the letters with a specific quality that would warrant the use of a proper noun, i.e. the Guantanamo Letters.

Thus, some common nouns can become proper nouns over time as they gain specificity in the minds of people. This can be a very organic process that can leave plenty of room for interpretation as to whether something should be considered a proper noun or not.