Sally (la Sal, the Salster) is part whippet, part Labrador and part dormouse. She is 16 years old, stone deaf, three-quarters blind and has dementia. She sleeps like the dead all day but loves her evening walk. We’ve decided that for as long as she enjoys her walks and remains continent indoors we’ll delay taking her to the vet and asking him to put her light out.

‘We’re talking about you,’ I shout at her after we’ve had a review because the dementia has become more obvious. No response. Deaf as a post. ‘You’re on borrowed time, sweetheart,’ I say, lifting her ear to speak into her head. No response. Strange it must be for a dog to live in silence. At one time she used to jump out of her skin and hide under the table every time I sneezed.

The sense of smell remains, though she must sniff more energetically than before for nuance. When out for the evening walk she likes above all else to check the dog messaging boards along the route. It’s her one remaining interest. The message board might be an unremarkable patch of long grass or a wall end but apparently there is as much information to be gleaned from one of these canine pissoirs as there is from a Sunday newspaper after a stunning by-election result. She trots from one of these places to the next fanatically inhaling the myriad urine signatures. If we deviate from the usual route she sulks for hours.

Pre-dementia, she maintained a proper perspective on the sensational stuff she was getting from these rank spots. It was the message, not the medium that was of primary importance. She bore in mind that however fascinating a tale told by the commixture of urines, the bigger picture was that she was out for a walk. And the even larger picture was that the longer she kept him waiting around the next corner for her to catch up, the more likely the foolish old man with the lead tied around his waist was to threaten her with violence. A one-minute pause at each message board was the tacit agreement. Time enough for her to read the small print and footnotes, and roughly the length of time before my mental self-sufficiency expired. After that we would both move on with interest and self-esteem intact.

The Salster’s creeping dementia has caused her to rip up that agreement. Now the medium is the message. Once she’s in that piquant, ammoniac zone the world is not enough and there’s no dragging her out of it. When I retrace my steps to remind her about this flaming walk we’re supposed to be on, it’s no longer a case of: ‘Sorry about that. I was quite carried away, you know. Very interesting new poster. I can assure you it won’t happen again.’ Now it’s: ‘Bugger off. I’ve found happiness and I’m not going anywhere.’

Walking on and hoping she’ll come to her senses and run to catch up? Tried that. When she comes out of her trance, she’s forgotten who she is and where she is and which way is up and I have to go searching for her down in the village. Or she’ll take up with the next person or persons to come along the path, presuming they are me, and get lost that way.

But she’s a harmless old thing and before dementia’s miserable twilight set in la Sal had the sunniest, most engaging smile of any dog I’ve known. She hated to be touched and never sought affection but allowed children to stroke her ever so lightly. Before she lived here in the Midi, home was the second floor of an apartment in Glasgow. I’ve read somewhere that Glasgow has the second thickest cloud cover in the world. Sally has never quite cottoned on to the fact that down here we have a sun, and that this sun’s heat in high summer is unbearable for every living creature except perhaps the soldier ant. Unaware that she’s moved from Scotland to the south of France, she will lie out in 40 degrees on terrace tiling too hot for a bare foot without noticing the difference in climate.

Now we’ve reached the stage where she’s lost her short-term memory and forgets she’s had her tea and asks for another. If you give her another tin out of compassion because she’s not long for this world, she’ll forget that she’s had that one too. And often she’ll stand motionless and stare at the wall for a very long time as if she knows she’s reached an ending. The other evening I sat on the floor and stared with her at the same spot on the wall. And after a while she turned and looked at me so sadly. Dear Sal.