Howard Jacobson: Life without certainties is good for your health - but don't take my word for it

I would be interested to know if cardiologists have more to do when war flares in the Middle East

So what happened to uncertainty? Where did still making your mind up go, or not knowing what you believe, or, even better, not wanting to believe anything? Keats's negative capability - "That is when a man is capable of being in uncertainties, mysteries, doubts, without any irritable reaching after fact and reason."

I take "irritable" to be the surprising word there, implying that striving after certain knowledge is against our better interests, not only as poets but as men. Uncertain, we are more creative, that goes without saying. The new thought is that, uncertain, we are calmer, too. I do not, of course, offer that as a certitude. But I ask you to entertain this possibility: not knowing what we believe, owning up to doubts, trusting to what is not clear - this negative capability (which, note well, does not render us incapable) is kinder on our tempers, on our hearts, and therefore on our well-being.

There is something else to be asked as regards the intemperate outpouring of opinion and conviction which world events - let's pick the Middle East at random - provoke: what happened to keeping your views to yourself?

There's a sentence of D H Lawrence's, probably from a letter, that lodges at the back of my mind - "Every heart has its secrets", or something like. It's a hands-off notice, partly. A refusal of intimacy and confidence. But if it's an assertion of the inviolability of feeling, it is also an assertion of its privacy. You owe your heart its seclusion, but you owe it to others, no less, to sometimes spare them your heart's meditations. In a new book, entitled The Heart Speaks, the cardiologist Dr Mimi Guarneri puts it less wistfully. "Each heart has its own biography, language and method of revealing its truth."

I'm with Lawrence and Mimi Guarneri. You have to leave the heart to make its own transactions. In the end the heart knows what's best for it. When to be at peace, and when to break. As a boy I used to fancy I would die of a broken heart, and I can imagine circumstances in which I still might. Stress cardiomyopathy, it's called in the business, or broken heart syndrome. The production of so much adrenalin as a consequence of fear, anxiety or loss, that the heart cannot bear it. Loss was my bag. I prefigured the anguish of being left to grieve by one I loved, and practised dying of a broken heart.

Dr Guarneri identifies another cause of cardiomyopathy. Hostility. Aggression, according to her researches, attacks your heart. Surprise surprise. As though, in the act of shaping hostile thoughts, we do not feel the strain our heart is under. But she has a remedy. Forgiveness, gratitude and optimism. I wish her well in her endeavours. Forgiveness, gratitude, and optimism? Not on this planet.

My own view is that negative capability is our only help. It is hard to be hostile when we're busy not knowing what we think. Certainty is what kills. We know that from our emotional lives. After a bereavement it is the certainty that we will never again see the person we love most that breaks our hearts. How often do we find the evidence of such heartbreak inscribed on headstones, one spouse quickly following the other into the grave, the absoluteness of death being more than can be borne. "Thou'lt come no more," says Lear of Cordelia. "Never, never, never, never, never." After which realisation of utter, unchangeable finality, he cannot live.

And the certainty component of hostility when it comes to political opinion is surely just as toxic. I would be interested to know whether cardiologists have more to do whenever war flares in the Middle East. Cardiologists here, I mean, a long way from the battlefront. For it cannot be that people will deliver themselves of such absolutist bile and their hearts not be the worse for it. I say people but in fact I mean journalists, ideologues, and other professional hunters after certain knowledge. Out on the street, around dinner tables and at garden parties, no one is saying much. Some instinct for self-preservation tells us to keep our heads down, the deliberations of our hearts private, and find other topics of conversation. Not because we fear what we might provoke in others, but because we fear what we will provoke in ourselves. And besides, out on the street, we are not sure anyway what it is we want to say. The man on the Clapham omnibus is never more indispensable than when he lacks a view.

Back in the media world, or in those everyman-a-blogger vituperations which some online newspapers encourage, opinions are as plentiful and lethal as missiles. Though commentators and the mad-dog pack that follows them gesture at the tragedy of events, it is not as tragedy - ineluctable, impervious to blame or censure, that which could never have been and never will be otherwise - that they treat it. They call it tragedy, beat their breasts and rend their garments, note the cyclical nature of its repetitions - his eye for your eye, your eye for his, until at last everyone is blind - then get on with the blaming game.

I have read such things in the past fortnight, not exclusively against Israel but primarily so - and no doubt in other parts of the world where sympathies are distributed more evenly I would hear comparable vilenesses spoken against the other side - as would make the angels weep. Has there ever been - by such accounts - a more evil power in the history of humanity than Israel? Has any country ever demonstrated so great a disregard for human life, or evinced such racist contempt for its enemy, or been so bent on the destruction of innocence wherever it can be found and blown apart? Did any people ever go so far out of their way to destroy, for no justifiable reason, the lives of their neighbours? Were so many war criminals ever before assembled in one place?

I say, and say again, I do not doubt that judgements no less extreme are made of Hizbollah elsewhere. And I am not concerned to refute any charge against anyone. I only ask what it must feel like inside the chests of those in whom hatred is so obdurate that they condemn without uncertainty or compunction. Imagine being able to pick your way through the labyrinth of confused event and embittered time with such blazing exactitude, knowing every lie from every truth, never pausing to wonder or extenuate, to weigh and understand, to doubt and sorrow. Pity, I say, their poor hearts.