Halliday & Matthiessen (2004: 326):
Mass nouns representing abstract things, and also things which are concrete but general, often move into the count category … . We could think in terms of a cline of countability, ranging from those nouns (and pronouns) which construe things as fully itemised, at one end, to those which treat them as totally unbounded at the other. Typically, living beings and concrete objects are itemised, abstract entities (and nominalised processes and qualities) are unbounded, with institutions and collectives falling in between. But the distinction is made in the grammar, so the same entity may be construed in more than one way … .