Visit the Francis Bacon exhibition at the Maillol Museum in Paris. Read Susan Sontag's latest book, Regarding the Pain of Others. The exhibition, despite the stupid subtitle of Sacred and Profane, represents succinctly a long life's work. The book is a remarkably probing meditation about war, physical mutilation and the effect of war photographs. Somewhere in my mind the book and exhibition refer to one another. I'm not yet sure how.

As a figurative painter, Bacon had the cunning of a Fragonard. (The comparison would have amused him, and both were accomplished painters of physical sensation - one of pleasure and the other of pain.) Bacon's cunning has understandably intrigued and challenged at least two generations of painters.

If, during 50 years, I have been critical of Bacon's work, it is because I was convinced he painted in order to shock, both himself and others. And such a motive, I believed, would wear thin with time. Last week, as I walked backwards and forwards before the paintings in the Rue des Grenelles, I perceived something I'd not understood before, and I felt a sudden gratitude to a painter whose work I'd questioned for such a long while.

Bacon's vision from the late 1930s to his death in 1992 was of a pitiless world. He repeatedly painted the human body or parts of the body in discomfort or want or agony. Sometimes the pain involved looks as if it has been inflicted; more often it seems to originate from within, from the guts of the body itself, from the misfortune of being physical. Bacon consciously played with his name to create a myth, and he succeeded in this. He claimed descent from his namesake, the 16th-century English empiricist philosopher, and he painted human flesh as if it were a rasher of bacon (tranche du lard fumé).

Yet it is not this that makes his world more pitiless than any painted before. European art is full of assassinations, executions and martyrs. In Goya, the first artist of the 20th century (20th, yes), one listens to the artist's own outrage. What is different in Bacon's vision is that there are no witnesses and there is no grief. Nobody painted by him notices what is happening to somebody else painted by him. Such ubiquitous indifference is crueller than any mutilation.

In addition, there is the muteness of the settings in which he places his figures. This muteness is like the coldness of a freezer which remains constant whatever is deposited in it. Bacon's theatre, unlike Artaud's, has little to do with ritual, because no space around his figures receives their gestures. Every enacted calamity is presented as a mere collateral accident.

During his lifetime, such a vision was nourished and haunted by the melodramas of a very provincial bohemian circle, within which nobody gave a fuck about what was happening elsewhere. And yet ... and yet the pitiless world Bacon conjured up and tried to exorcise has turned out to be prophetic. It can happen that the personal drama of an artist reflects within half a century the crisis of an entire civilisation. How? Mysteriously.

Has not the world always been pitiless? Today's pitilessness is perhaps more unremitting, pervasive and continuous. It spares neither the planet itself, nor anyone living on it anywhere. Abstract because, deriving from the sole logic of the pursuit of profit (as cold as the freezer), it threatens to make obsolete all other sets of belief, along with their traditions of facing the cruelty of life with dignity and some flashes of hope.

Return to Bacon and what his work reveals. He obsessively used the pictorial language and thematic references of some earlier painters - such as Velásquez, Michelangelo, Ingres or Van Gogh. This "continuity" makes the devastation of his vision more complete.

The Renaissance idealisation of the naked human body, the church's promise of redemption, the Classical notion of heroism, or Van Gogh's ardent 19th-century belief in democracy - these are revealed within his vision to be in tatters, powerless before the pitilessness. Bacon picks up the shreds and uses them as swabs. This is what I had not taken in before. Here was the revelation.

A revelation that confirms an insight: to engage today with the traditional vocabulary, as employed by the powerful and their media, only adds to the surrounding murkiness and devastation. There are a number of words and cliches, filched from the past, whose currency has now to be categorically refused. Liberty, terrorism, security, democratic, fanatic, anti-semitic, etc are terms that have been reduced to rags in order to camouflage the new ruling pitilessness.

This does not necessarily mean silence. It means choosing the voices one wishes to join. The present period of history is one of the Wall. When the Berlin one fell, the prepared plans to build walls everywhere were unrolled. Concrete, bureaucratic, surveillance, security, racist, zone walls. Everywhere the walls separate the desperate poor from those who hope against hope to stay relatively rich. The walls cross every sphere from crop cultivation to healthcare. They exist, too, in the richest metropolises of the world. The Wall is the front line of what, long ago, was called the class war.

On the one side: every armament conceivable, the dream of no-body-bag wars, the media, plenty, hygiene, many passwords to glamour. On the other: stones, short supplies, feuds, the violence of revenge, rampant illness, an acceptance of death and an on-going preoccupation with surviving one more night - or perhaps one more week - together.

The choice of meaning in the world today is here between the two sides of the wall. The wall is also inside each one of us. Whatever our circumstances, we can choose within ourselves which side of the wall we are attuned to. It is not a wall between good and evil. Both exist on both sides. The choice is between self-respect and self-chaos.

On the side of the powerful there is a conformism of fear - they never forget the wall - and the mouthing of words that no longer mean anything. Such muteness is what Bacon painted.

On the other side there are multitudinous, disparate, sometimes disappearing, languages with whose vocabularies a sense can be made of life even if, particularly if, that sense is tragic.

   "When my words were wheat
   I was earth. When my words were anger
   I was storm.
    When my words were rock
   I was river.
   When my words turned honey
   Flies covered my lips".
   - Mahmoud Darweesh

Bacon painted the muteness fearlessly, and in this was he not closer to those on the other side, for whom the walls are one more obstacle to get around, even if it involves risking their lives for those following? It could be ...

· Francis Bacon: Sacred and Profane is at the Maillol Museum, Paris, until June 30. Details: 00 33 1 42 22 59 58.