It is customary when discussing documentaries on mortality to discover an optimistic message amid the misery. It is to the directors’ credit that they make no efforts to cook up inspiring fillips in their study of a death from motor neurone disease.

I Am Breathing does offer us an inspiring, amusing hero in Neil Platt. Diagnosed with the condition at 33, the Scottish architect declined comparatively rapidly. Throughout his illness he worked on a blog and collaborated on this very respectful film.

But for all the warmth of Platt’s personality and the courage of his even-tempered wife Louise, this remains a properly depressing piece of work. There are few revelations about eternity. There is no great getting of wisdom. The film remains brutally honest about the finality of finality.

This is not to suggest that I Am Breathing is without its humorous moments. Hung around the subject's own words, the film takes us through a life which, though comfortable, was in most regards quite ordinary. Neil enjoyed a drink and a smoke. He treasured a battered leather jacket bought at the Cornmarket in Edinburgh. He and Louise had a child shortly before symptoms began to show. One day, he found that he'd lost some control of his foot. Within months the debilitating sclerosis had set in.

Making judicious use of ambient throbs from Kieran Hebden and Jim Sutherland, the film allows its subject to compose a kind of multi-media epistle to his young son. Words appear on the screen detailing his inexorable decline. Neil places certain items – that jacket, a Zippo lighter – into a special box. The sense of a life being defined by its imminent end becomes increasingly overpowering as the film progresses.

Perhaps there is some good news here. The Platts prove that it is possible to face up to the most appalling challenges with dignity. Neil also has a message for us: don’t take your time for granted. Moving words.