When an uncountable noun has an adjective before it or a modifying phrase after it you can use an article. You can also leave it out – both are correct and there is no difference in meaning.
- He had a rare courage.
(article/a – adjective/rare – uncountable noun/courage)
He had rare courage.
(no article – adjective/rare – uncountable noun/courage) - He had a courage not often seen.
(article/a – uncountable noun/courage – modifying phrase/not often seen)
He had courage not often seen.
(no article – uncountable noun/courage – modifying phrase/not often seen)
