There can’t be many people who realise their dad has Alzheimer’s from listening to the BBC’s Today programme. But six years ago, hearing my brilliant and erudite father, the Liberal Democrat peer Lord Goodhart, stumbling and pausing through his interview with John Humphrys, I knew something was very wrong. Not that being reduced to a stuttering wreck by Humphrys is necessarily a sign of dementia – if it were, we’d have very few senior politicians left – but that sort of thing didn’t happen to my dad. He was calm, clear-thinking, unflustered and always in full possession of the facts. He knew his onions. He was that frowned-on creature these days – an expert.

If that sounds braggadocious, well, yeah. It is. But my dad died in January, and I feel like bragging about him. He was the kind of man who deserves to be bragged about, although it is the last thing he would have dreamed of doing himself – he was modest to a fault.

That was the last of many appearances on Today – someone in an office somewhere on Portland Place would have quietly put a red “x” through his name, another poor old fellow watching helplessly as his marbles gradually rolled out of reach. Although Dad didn’t officially stand down from the House of Lords until later, in many ways, that interview marked the end of a long and distinguished career of public service.

It was a career that encompassed a successful legal practice, as well as chairing the human rights organisation Justice, and travelling overseas, as far afield as Kashmir and Sri Lanka,to report on human rights violations. He wrote the constitution for both the SDP and the Liberal Democrats. In 1997, he was made a lord, and promptly set about trying to put himself out of a job by campaigning for a largely elected second chamber. As the Lib Dem shadow lord chancellor, he was steadfast in his assertion that the war in Iraq was illegal, just as he was unequivocal that there was never any justification for torture. He was – to my mind – on the right side of every argument. But then, don’t most kids think that about their dad?

Lord Goodhart canvassing.

For us – my sisters, my mother and me – the door closing on Dad’s career marked the beginning of a new era: one that was by turns agonising, baffling, heartbreaking and, I must confess, comedic. Alzheimer’s affects everyone differently, including those around them – but the ability to laugh at its quirks and peculiarities sustained us all in the darkest times.

For almost 50 years of married life, Dad got up in the morning and brought Mum a cup of tea in bed. This came to a rather abrupt halt the morning her cup of tea consisted of orange juice, milk, and some potted shrimps all stirred together. My mother demurred, though Dad consumed his with alacrity and chided her for being fussy. At times like that, it is easier to laugh than to cry.

On another occasion, two years ago, Dad took me and my wife to the opera. An opera devotee, it was his last visit: as the lights went down for act three of a rather lengthy German comic opera, he called out in despair, “Oh God.” A few minutes later, he heckled (I suspect a first for the rarefied audience): “Get on with it!” My wife and I, being of reasonably sound mind, were inclined to agree. That was also the evening Dad looked at his diary, which he did every five minutes, for reassurance, and read “To opera with Benjie.” Then he looked up at me. “Are you Benjie?”

He also cast off some of his inhibitions. He would happily sit and hold hands, and began calling me darling

That’s what it’s like, though, watching someone you love journey down Alzheimer’s meandering path: a series of minor heartbreaks. You lose them by increments. First, they are struggling to cope with the finer points of policy on Radio 4. Then they can’t remember the word for rain, so say “water falling from the sky”. Then they can’t go to the bank, because they try to use their diary in the cashpoint machine. Then they can’t go out alone. Then they forget your name, and how to use a spoon, until finally they don’t even recognise your face, and can’t really walk, and have forgotten the word not just for rain, but for wife and daughter and son and bed and everything else. It takes seconds to read, but years to happen.

It was worst of all for my mum. She had spent pretty much all of her adult life being dazzled by a man of towering intellect. I mean, obviously she wasn’t permanently dazzled – sometimes she was just bloody annoyed, as he reloaded the dishwasher to his exact specifications, or took her round yet another Renaissance church – but she loved his brilliant mind. He was the only man I ever saw who would consistently do better in Mastermind on people’s specialised subjects than they would. She watched it all slip away, and watched my strong, brave, independent, free-thinking father become an uncomfortable, ill-at-ease figure who followed her from room-to-room, checking and re-checking his diary for reassurance. What results, inevitably, is not just grief, but irritation, guilt, and (entirely unnecessary) self-abasement.

In the autumn of 2015, Dad went into a nursing home. By now, his dementia was progressing rapidly. We had been told that we would all know when it was time to find a home for him, and so it proved. Oddly, and against all my preconceptions, I developed a marked fondness for the place. Dad was beautifully looked after, and I think found a measure of contentment there that none of us had expected. Perhaps being removed from his old life allowed him to come to terms more easily with his new reality.

To my delight, he also cast off some of the inhibitions that are a feature of so many men of his generation. He would happily sit and hold hands, and began calling me darling. Of course, this was in part because he had forgotten my name but, still, I enjoyed it. He even developed an unlikely fondness for You’ve Been Framed!. (I also once found him with the Daily Mail in his room, at which point we knew things were bad.)

Those last months of Dad’s life were not without their consolations, and one superseded them all: Alzheimer’s cushioned my dad from some political realities that would have broken his heart, if not his resolve. The annihilation of the Lib Dems at the 2015 general election left a vacuum at the centre of British politics just when the party was needed the most. As a half-American who cherished his transatlantic roots, he would have despised everything that President Trump represents. As an enthusiastic European, he would have abhorred Brexit. And as a thinker and intellectual, he would have loathed the wilful misinformation and visceral insularity that both campaigns embodied. But as the world he had fought for with such passion and zeal turned to dust, he was in the garden of the care home, happily counting the aeroplanes that passed overhead.

On Dad’s death certificate, the cause of death, along with Alzheimer’s, is urosepsis. If he is looking down now from the great debating chamber in the sky (which, as a lifelong humanist, would come as some surprise to him), he would insist that the word was missing an “e” at the beginning. And then he’d laugh.

On the morning that Dad died, my mum, my sisters and I were there with him, along with the saintly carer, Cathy, who had looked after my parents through their darkest times. In the care home, they placed a series of blue paper butterflies on Dad’s door, to let others know that some privacy was needed in his room as the end neared. It was a simple and deeply moving gesture, heightened by the fact that Dad had always loved, and could instantly identify, all types of butterfly.

An hour after he had died, peacefully and in the presence of people who loved him as he deserved, the night shift staff came in and whispered their quiet goodbyes to Dad. Then the day shift came in and did the same. When they spoke to us, many were in tears, and every one of them mentioned how polite, courteous and kind he was, and how fond they had been of him. My dad had hung on to his warmth and decency, even when everything else had left him. It was just another in a long, long list of reasons to be proud of him.