Halliday & Matthiessen (2014: 385):
Things are represented in English as either (a) discrete, and therefore countable, or (b) continuous, and therefore uncountable; the grammar thus makes a categorical distinction into count nouns and mass nouns, such that count nouns select for NUMBER: singular/plural, while mass nouns do not. As pointed out above, mass nouns are treated as singular where the deixis is specific, e.g. do you like this poetry/this poem?, and as plural where the deixis is non-specific, e.g. I’ve written some poetry/some poems.
The distinction is not quite as clear-cut as this suggests. Mass nouns are often itemised, and hence also pluralised; the meaning is either ‘a kind of’, as in I’ve found a new polish, or ‘an amount of’, as in three coffees please. There will then be an agnate expression having a measure/type word as Head: a new type of polish, three cups of coffee.