I think I saw from a couple of posts here that seems to vaguely indicate that an uncountable noun can be in plural depending on the perception (discreton??) of the writer at the time of writing. I think someone kind of alluded to the idea of writing (or talking) about it generally or as individual examples.

success and successes

happiness and happinesses

(I got two words from the posts here.)
Can an uncountable noun be plural? I do not think so, but some words can be either plural or uncountable. Just like the word 'plural', it can be uncountable or countable.

Happiness is an uncountable noun. You cannot count on happiness.
According to Collins Cobuild English Dictionary for Advanced Learners, 'success' is also a countable noun. One strong point about this dictionary is that it provides the different parts of speech of a word you are looking for and, if the word is a noun, it states whether that noun is countable or uncountable.

The illustrative sentence provided is 'We hope it will be a commercial success.'
Site Hint: Check out our list of pronunciation videos.
an uncountable noun can be in plural

No, not really.
The important point is that a noun in itself is not countable or uncountable. It is the usage of the noun that makes it so. No matter what a dictionary states as the countability of a noun, there will probably be someone somewhere who will use it in a different way. The dictionary only tells you the way the word is usually used, not the way it must always be used.

success, for example, can be thought of as a general state experienced by successful people (uncountable).

We all strive for success in everything we do.
Sometimes it is difficult to achieve success.

Or it can be thought of as an individual successful performance of an action. (countable).

Mrs. Dolling has had many successes in her long career as a concert violinist.
The bake sale raised a lot of money for disabled children. Everyone felt it was a success.

So it's not that an uncountable noun is being used in the plural (or with a/an); it's that plurality (or a/an) marks the noun as countable.

Thank you.
Thank you.
Students: Are you brave enough to let our tutors analyse your pronunciation?
Thank you.

If someone who proclaims or think himself to be proficient in English chooses to use an uncountable noun in the way that normally a person would consider can be used for countable nouns (in a way that is countable), then would you say that use is also valid and can have the(a??) stamp of approval from a grammarian of great prestige?

(Which is better, a or the? My rationale for using the is send the impression that what I meant to say is that it is a special (more or less the one) and definite stamp of approval.)

Thank you.
Hello CalifJim

You are talking about what is obvious.

I say that according to Collins Cobuild English Dictionary for Advanced Learners, 'success' is also a countable noun. This means it is both a countable and uncountable noun depending on how it is used. The point is that 'happiness' is an uncountable noun; there is no such word as 'happinesses'. On the other hand, 'success' as an uncountable noun is 'success', but as a countable noun is 'successes'. However, one must be careful to use both words correctly. It is unlike other countable nouns like apple, orange, table, etc.

The person who asked this question wanted to know whether 'happiness' and 'success' are countable nouns. I believe he now knows the answer to his question.