At mealtimes three of my cats eat on the floor, but the oldest, The Bear, eats on a chair above them. The Bear is 17 now, missing several teeth, and likes to eat at his own sedate pace, carefully sucking the jelly off each chunk of mechanically recovered meat. He pauses to glance up nervously every 15 seconds or so, as if paranoid an adversary might be creeping up behind him holding a Post-it note with the word "wanker" scrawled on it. He gets his food first, and I stick around until he's finished, making sure none of the others muscle in on him.

Some might say I'm spoiling him, but I see it as a fundamental part of the conditions I agreed to when I decided to live with my particular set of cats, just as, had you decided to take in three salamanders and Brian Sewell as pets, you'd find it necessary to segregate a little area purely for Sewell so he had space to work without being disturbed.

The Bear is, by some distance, the most polite cat I have ever met. He has never to my knowledge started a fight with a contemporary, or made any cliched or obnoxious demand for attention. He is the only feline I have ever known who signals his hunger not by cursing, miaowing or using my leg as a scratching post, but by nodding subtly towards the food cupboard. I found him with his first ever dead mouse not long ago, but I suspect one of the others had killed it and, having mistakenly thought it was still breathing, he'd moseyed over with the intention of reading it some elegiac verse as a send-off.

The Bear has more of a mioop than a miaow: a noise that, despite its gentle nature, can pull noisily at your heart strings with its central question, which usually seems to translate as, "Can you tell me why I am a cat, please?" This should not be confused with his special, repetitive piss miaow: a kind of warning system that is a little like a smoke alarm, but for urine instead of smoke.

The Bear was named for his resemblance to a teddy bear, but, the more you get to know him, what he becomes most reminiscent of is a sad owl. Looking into those his eyes, I feel I can see all the world's pain. "What is it, The Bear?" pretty much anyone who has ever spent any significant time with him will soon be tempted to ask. He'll answer only with another bereft look. While others shout and swear their way through the day, he continues to offer his silent commentary: all-knowing, wry, dignified, troubled.

I cannot think of many revelations about my life that would make me feel more unmoored than if someone were to find a way to measure cat IQs and reveal to me that The Bear is a simpleton. I've been acquainted with this cat for over twelve years now, and I feel I know his intellectual powers. Even if those soulful peepers signify nothing and mere coincidence explains his disappearances in the build-up to every house move I've ever had, or his eerie way of gravitating towards me every time I'm ill or sad, you cannot doubt that he has had the most character-building of cat lives: all nine of the standard ones, that is, plus the seven or eight bonus ones he seems to have been granted.

The Bear (a cat) Photograph: Tom Cox

He was originally found in a plastic bag on the hard shoulder of a motorway, along with several of his siblings. Since then, his fur has all fallen out due to a flea allergy then all fallen out again due to an allergy to flea treatment medicine. He's withstood carbon monoxide poisoning, had a hole ripped in his throat by a feral challenger, developed asthma, lost chunks of both ears, gone awol for almost six weeks in south London, moved house a dozen times, and been rather brutally given his marching orders on countless occasions by Biscuit, my nextdoor neighbours' cat, whose Last Of The Summer Wine affection he pines for.

The Bear is also the moggy of a broken marriage. He was originally not just my ex's cat, but ex's ex's favourite cat. I got to keep him when my ex and I broke up, as we agreed that he got on better with me (for example, he had never neatly deposited a turd in the pocket of my freshly laundered dressing gown).

The Bear's recent life has been more content and calm, though he has still been known to stealthily vent his frustration by pissing on my Bill Withers LPs. Sometimes he'll stray a bit to the right, and hit XTC, or a fraction to the left, and catch the West Coast Pop Art Experimental Band, but it's mostly Bill who gets the brunt of it. It's upsetting, but I see it as his way of putting forward the very reasonable argument that nobody really needs nine Bill Withers LPs.

On the whole, The Bear has come a long way since those early days as a small black cloud on legs looking for somewhere to rain. He looks plusher than ever - it's a long time since his hair last fell out - and is far more accommodating than he once was to house guests, although the beeline he makes for those with a history of depression or a penchant for 19th-century gothic literature has been noted.

But in the last few months, I've noticed the difficulty The Bear has climbing down from any object higher than his own head. The customary camp wobble to his walk is starting to get more wobbly, but in an arthritic way. His back feels worryingly brittle when I stroke it. He'll sometimes shuffle towards the cat flap, see that it is raining outside, then look up at me and turn around, as if to say, "Oh, it's all a bit too much. I don't think I'll bother."

The Bear (a cat) Photograph: Tom Cox

I've perhaps believed him, more than any of my other cats, to be invincible because he's withstood so much, but I should probably prepare myself for the possibility that The Bear might not keep going strong for another 22 years and become the world's oldest ever feline.

When I try to imagine The Bear not being here, it's tough. I find it easier to imagine myself with an entirely different name and career, living in a country I've never visited. I love each of my cats equally, and for different reasons, but my relationship with The Bear is unique: not so much like a bond with a cat at all, but the kind of attachment you might have to a mute friend you'd met in a hostage situation, and who'd been burdened with the job of feeling every emotion twice as acutely as anyone else.

"Wow, man," said my hippie pal Michael, who once looked after The Bear for a few weeks. "When I first cuddled him, that was intense. I've not felt anything like that before." I knew precisely what he meant.

Maybe The Bear did go through a stage where he had the capacity to be swaggering and indifferent, like my other cats, or maybe his eyes saw too much too soon for that to be possible, and then just kept seeing more. There's a lot of soppy pet memoir nonsense I don't buy about animals "teaching you" stuff, but living with The Bear is a lesson about ageing and battle wounds.

Yes, he could be young and scar-free. He could still have perfectly intact ears and the confidence to hurtle up a tree, not believing anything bad could happen to him, or rip a vole's face off. And that would be great for him in many ways. But what wisdom would he lose in the process? What enigma, which layers? Would he be quite as interesting to be around? And - most crucially of all - could you then still say that he was truly The Bear?

• Tom Cox is the author of Under the Paw: Confessions of a Cat Man and Talk to the Tail: Adventures in Cat Ownership and Beyond. Follow him on Twitter at twitter.com/cox_tom.