I've always wondered what is the correct use of 'uncountable nouns', for example 'Water', we might see a use like this:
'There is water over here'
This is different to a 'class' or 'category' name like 'pen' where we would say 'There is a pen over here'.
This suggests to me that 'Water' is more equivalent to 'a pen' then to 'pen'.
It therefore follows that such 'uncountable nouns' may have the ability to denote different objects as indefinite descriptions can, unlike 'class names' such as 'pen' which denote an abstract idea.