'Joe, I suspect, has no sense at all that he was born, or will die'

Our ideas about people who are severely disabled raise tough questions of what it means to be human.

Midday, and I've just taken Joe for a drive, turning left and right at his languid command, follow- ing his pointed directions while he completes another piece of that intricate jigsaw, his apparently flawless mental street map of surrounding districts. He remembers his way better than I. He's 11, autistic and severely developmentally delayed, but right now "mental impairment" is not a phrase that springs to mind.

It's undeniable, though, that Joe is a strange case. We've been cruising here and there for about an hour by the time we pull up outside the house. I coax him from the car, he slouches up the path, we step inside. With a sigh, he settles cross-legged into the big red armchair, stares into space for the briefest of moments, and issues his next instruction:

"Car."

"What?"

"Car . . . Dee [his word for daddy, from its second syllable]. Car!"

"But Joe," I say pointlessly, beyond his understanding, "we spent the last hour in the car."

"Dee! Car!"

"Joe, you cannot be serious."

"Dee! Car!"

It's his latest obsession: to drive, listening interminably to a handful of cassettes that begins and ends with Albinoni, Vivaldi, the Jam and Abba. Of late, even the Jam has been passing out of favour, chucked last weekend to the back seat. As we drive, Joe looks with that curiously still and contemplative sideways gaze that seems to soak up detail in a way that few "able" children - and not many adults either - could begin to comprehend. It's clear this fixation would keep me at the wheel all day if Joe had his way, and the mental map-making would stretch who knows how far beyond Hertfordshire; Inverness, say.

If you were even mildly environmentally concerned and found your 11-year-old to be an accidental petrolhead, it would be bad enough, but you'd have worse to come: Joe's obsessions possess him utterly. When I tell him we'll take another drive at five o'clock (five hours away), he pulls up a chair in front of the kitchen clock, sits down to stare at the passing minutes and waits, and points, and asks constantly for reassurance that the moment will come. His understanding of time is far from complete, but he recognises the numbers on the clock face and his priorities are clear: given the alternatives that day, there is no realisable purpose or pleasure capable of supplanting the car, or even interrupting his long - sometimes very long - anticipation of its meandering bliss. By comparison, Jeremy Clarkson is an Amish.

Joe is seldom at his worst; he is often calm, affectionate, fascinating and funny. But life for those close to him when he does slip into that mania is to be the pinball in an all-day hammering from one headstrong obsession to another that sometimes turns violent. I laugh about it as an antidote to collapse. Any relationship has its weirdness; we all have eccentricities others must bear, but it is fair to say that Joe's tend to the more testing. And the test is about more than just forbearance, because Joe poses altogether tougher questions.

You can run through the philosopher's common measures of what it means to be one of us and find either that Joe fails them outright, or that his inclusion is in doubt. Humanity has deep structured language, it is said; Joe does not. Human beings have complex morality; Joe demonstrates time and again, sometimes brutally, a frail grasp of moral norms or instincts, mostly because he lacks an adequate appreciation of how his behaviour affects others. We have rich self-consciousness; Joe has little if any concern for how others see him and scant reflection, I suspect, on his own thoughts. I can never be sure, and others who know him well think my doubts pessimistic, but it is not a settled fact for me that Joe is really self-conscious with the richness most of us understand of that concept. It is also said that people are, above all, social creatures whose relationships are uniquely subtle and sophisticated; Joe, according to the dominant theories of autism, might be unaware that other people have any kind of mental life, might be blind to the existence of others' minds, and thus incapable of understanding their behaviour or making sense of social situations.

If that paragraph makes you wince, you're not alone. When I wrote a book about Joe that included a brief reflection on his humanity, it provoked one or two shudders, or else people used words like "brave". Though my conclusion was never in doubt they asked, "How could you, his father?"

These are understandable reactions, but misconceived. What they neglect is the way the world merrily invites questions about human worth, all the time. To take just a couple of examples: whenever anyone considers a termination for fear of having a disabled child, there lurks - among other painful considerations - doubt about the case for a single, comprehensive category of humanity. I don't make light of the difficulty of such decisions, but they raise irresistible and awkward questions for all of us. With certain decisions that the elderly are less a priority for health or social care than the young, we continually make judgements about relative human worth, and sow doubt about general principles of equality. Humanity, this most collective of words, is clearly not, at least in modern practice, treated as an indivisible status. History, too, teaches us that at various times we have accepted distinctions that place some people into other, separate categories.

So I do not invent the challenge to Joe's worth. On the contrary, it seems to be often in my face. I could pretend it wasn't there, but may be incapable of letting the argument go. In my defence, there's some provocation: for it feels to me as if it broods over Joe like a vast cultural and moral question mark, in almost everything he does and we do together. Worse, the philosopher Peter Singer answers that question with ruthless logic, arguing that people without self-consciousness could be treated as we treat farm animals: the death of one such being is of little consequence, especially if death means nothing to them, and if they are replaced by another. When reading Singer's argument, I am painfully aware that Joe, I suspect, has no sense at all that he was born, or will die.

That Singer might have been content to see Joe done away with - on the grounds that others could make better, more appreciative use of the resources he consumes - leaves me feeling, as you might imagine, belligerently protective. And perhaps that fighting talk is why no one expects a serious reply from me.

But if we take that expectation, too, to its logical conclusion, it leaves us in a strange place: the odd corollary is that to love is to be disqualified from argument, or, just as bad, exempt from the need for reason. One reviewer, for example, objected to my inclusion in Joe's story of what she disdainfully called "theory". I suspect that we don't fully appreciate quite how perverse this demarcation is in a discussion of human nature. So accustomed have we become to the principle that difficult arguments should ideally be dealt with dispassionately, that I wonder if we are sometimes tempted to stamp on the full breadth of the subject as well as its treatment. Have we extended the ideal into a realm where human passions ought to be a critical part of the territory? So readily do we accept that love is its own justification that we spare it our disagreement when we disapprove, and offer pity instead.

I find the separation of feeling from reason strange. Perhaps all that tells you is where Joe got his autism, but I know of no one who will admit to an anger without cause, a pride without foundation, a love utterly without reason. Heart and mind ought to be recognised more often for their unity than their conflict.

I have no qualification to talk about these issues, of course, other than being Joe's dad. I'm an amateur. But whereas most amateurs are said to be entitled to their considered if uninformed opinion, it is as if I can't be expected to take the discussion remotely seriously; as if I'm supposed to sit around awaiting the conclusion of other people's deliberations about my son's status in the world; as if I'm disqualified from offering a considered answer because I am - presumably - compromised by too intimate an involvement, compromised by love; as if we don't expect anything sensible from those to whom the question is most relevant. For some, I'm the kid in the corner who wasn't meant to be listening to the grown-ups' chatter about children.

It could be that I've been carried away by all this, being altogether too partisan, but I don't think so. I think that the thinking about it is in general muddled by embarrassment for me, and the muddle is exposed when it becomes apparent that one of the children was not just listening, but thinks some of the grown-ups have ignored what matters: that love, far from invalidating my argument as a result of my closeness to Joe, has to be the cornerstone of any argument about human nature.

Let's examine the claim that humanity is most uniquely characterised by its complicated social relationships, an argument advanced by the 18th-century philosophers before Darwin (as has been done again persuasively, more recently, by Nicholas Humphrey and others). This is a standard Joe seems ill-equipped to meet. However, we soon see that, by such a definition, his incapacity is only half the test, and his relative difficulties inconclusive. For relationships extend in more than one direction, and Joe is embroiled in human relationships, sometimes whether he likes it or not. The ones he shares with me, his mother and others who care for him are directed by us on - what else? - human terms. He lives among us, his needs are facilitated by us, he couldn't exist in any other way. The conclusion is that Joe is defined not only by his own biological capabilities, by his own genes, but by what I am and what others around him are, by how we behave with him and how we treat him, by the fact that we care for him.

As the writer Kenan Malik has said in making a sometimes lonely case against the tide of reductionism about human nature, humanity is not invested in a single person. It is a collective label describing our existence as social beings. It has meaning only in relation to others. And if we try to discuss it by excluding from the discussion those who form relationships with anyone at the margins, anyone who can't answer for themselves, we are likely to get it wrong. Defining humanity is a tricky business, but if we are embarrassed to hear from those who love, we rule out a most compelling part of the definition. This is an argument that applies with unique force to the question of humanity, and yet seems in this case also to arouse unusual squeamishness. If NS readers can tell me why, I'd be sincerely grateful. And if the reasoning here is right, then though we are in the habit of behaving as if humanity can be graded, we diminish ourselves each time we do.

This is one reason I think Singer so wrong. The rigour of his logic might apply if it were possible to treat each case as separate and distinct from everyone else, but nobody lives so isolated a life. No such world exists where the plight of one is measured regardless of what others think and feel. His insistence that we need to keep sentiment separate from logic is also the root of his error.

Even then, I think he neglects other important arguments, and, if anything I have said makes sense, I hope it will be granted that it is reasonable for me to make them.

One answer to the problem of how to address severe mental disability is that we regard these people as being like us because they once had the potential to be like us. Potential is a useful thought: it idealises humanity and claims there is a germ of the ideal in us all. That Platonic promise, though it be cruelly frustrated in the event, should nevertheless be taken as a determining factor in our identity. And if, by bad luck, potential is savagely unrealised, it should make no difference to our fundamental moral judgements of one another, because to some extent we all fall short.

Yet one must admit that potential has problems. It reminds me of the Jane Austen character who could have been a great pianist . . . if only she had ever learned to play. Potential, it may be argued, is nothing if not realised. If, as seems likely, Joe's problems are genetically based, he may never have had the potential to become like us, except before conception, when he existed as a nice idea. "It could have been different. If only it was different." Yeah, right. "I could have been a contender." Sure. We may regret what might have been, but we don't often end our judgement there. On the contrary, we forget potential surprisingly easily in favour of the pianist who actually plays.

The philosopher Mary Midgley writes: "If we contrast a world in which life is still going on, though it will soon cease, with one in which actual life is permanently extinguished, but the seed stores and the sperm banks remain untouched for ever, we cannot intelligibly say that the real value lies in the second. Potentiality matters only because of what will happen when it is actualised. Could we think of the blueprints as more important than the building, the mix than the pudding, the match than the fire?"

More important? No. And yet suddenly I feel an unreasonable sympathy for the match that won't strike. Midgley must be right that potential is neither the only nor even the most important test, but surely it plays a part. It draws on sentiments hard to extinguish: hope and expectation. Even if time has rendered them futile, their memory lingers, and memories make identities, too. We hold on to them with phrases such as "I once hoped". And even though the hope be dashed, it remains expressive of us, and of the child for whom we hoped, that we once held it. For me, the non-striking match belongs to the class of things that are meant to burn brightly, and the gulf between potential and unfulfilled event fills me with a pity almost too great to bear. Those seed stores for ever untouched are an image of agony, and their potential stays with us whatever their ultimate fate. Why define entirely by accident rather than design? There is, I think, a place for potential in the measure of others, far back in the past though it may have died. And if potential has been frustrated? Well then, "bad luck" is all the difference. This thin phrase, casual in everyday use, does not diminish Joe; rather, it shows how narrow is the gap between us.

But there is a further characteristic of potential that brings us back to where we began. Every one of those sentiments and arguments about it, in logic and feeling, ties Joe's potential to the hopes and expectations of the rest of us. It is yet another collective quality. Potential means nothing to Joe himself. How could it, as he can never know what potential he might once have had? The argument about it is one he doesn't understand and can't be said to care for. Its importance to Joe derives from its meaning to the rest of us. It matters to him only because it matters, I hope, to you, as it matters to me, and we define Joe not only by what he can understand or could have hoped for, but what you and I can understand or could have hoped for. Part of his humanity, and his potential humanity, is also vested in us.

That we care is not to be in a category exempt from justification, but one that must make its case. It is not cause for embarrassment or avoidance, but a sine qua non. So at five o'clock, all distraction exhausted, Joe and I fetch the keys, get into the car and go for a drive. Joe, as usual, gives directions and I, as usual, put Vivaldi on.

Joe: the only boy in the world is published by Profile (£12.99)

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