The Spanish Portrait: From El Greco to Picasso Prado, Madrid, until 6 Feb  In this exhibition of 87 paintings from museums across the world, it is possible to undergo an experience such as you're unlikely to have had before or elsewhere. As with most things special, it will depend partly on chance and partly on your readiness to face the unusual.  The chance is that there are not too many other people at the exhibition. And as for preparing yourself, it's a question of forgetting that you are in one of the long galleries of the museum of the Prado. Leave art history behind. If you wish, you can consult the meticulous catalogue of the show afterwards: The Spanish Portrait: From El Greco to Picasso.  Envisage nakedness and, at the same instant, darkness. What you sense is nakedness, what you see is blackness. To do this brings you close to the heart of Spanish painting, but forget painting! No one here is thinking about painting. Not even the security guards.  Walk down the length of the gallery under the gaze of the hundred women, men and children on the walls who were awaiting the future, part of which you represent as you walk past them. Some look formal; others hide almost nothing. You can sometimes peer around the edges of the formality to glimpse the particular kind of endurance or anguish existing behind it.  The Condesa de Vilches (1853) with her blue satin dress with flounces and her skin-creamed shoulders and winsome smile, already knows she will be exhausted after the sitting and that her tiredness will be like an empty theatre when every light is out.  These are lives you want to approach more closely. Do so. Joanna of Austria (mid-16th century). A slender and imposing woman about 35 years old. Very feminine but aquiline face. Only her ear reveals a vulnerability. She might stride in any direction at any moment, but never without calculation. She embodies a determination not of the will nor of ambition, but the determination of a perfectly controlled pain. I imagine the timbre of her voice being unexpectedly low. A little like Jane Birkin when she talks.  One century later, a middle-aged man who might be a barber or a surgeon or a bootmaker. His eyes offer no easy promises, yet they are reassuring. A man whose compassion for others has in some way protected him. His worn white collar, which absorbs his sweat, speaks as much as his forehead of experience. There is the taste of the salt of the earth.  A procuress (20th century), for whom life opened its swollen mouth as if she were a dentist who could extract the throbbing tooth.  An early 19th-century banker seated by his desk, holding a business letter. Competent - even aggressive, alert. His left hand on his thigh is the hand of a boy waiting to be given permission to leave the table. His eyes are his most surprising feature; they have seen, a number of years ago, the unspeakable. They have seen civil war.  As you walk down this corridor of gazes, you of course recognise many of the painters - Velázquez, El Greco, Goya, Solano, Picasso - but at this moment authorship is secondary. What is hanging in the air is a multitude of unspoken questions and answers emerging from a hundred lives.  The dwarf from Vallecas, holding a pack of playing cards and on the point of laughter, asks: 'Who are you?' As I approach him, I find myself remembering a few lines from Tom Waits's latest CD. 'And tell me how does God/Choose, whose prayers does he/Refuse, who turns the wheel,/Who rolls the dice...?'  A nobleman with a peculiar left shoulder (amputated arm?) and the intelligent expressive eyes of a stag (Velázquez's marvellous portrait of a stag is not here) places his hand on his chest and he announces: 'I do not know why... I still don't know why...'  Each one of the hundred people painted is looking at the future with a question or declaration. We are walking between their life-experiences in a manner that could never happen if we were walking past photographs, however masterly the photographer. Photos are taken by surprise or take us by surprise. In photos, there is very little waiting-to-be-seen; photos are not attendant. Here, in the long gallery, there is nothing else but a waiting-to-be-seen. Therein lies the nakedness.  A two-year-old prince makes a bell tinkle. The bell is much smaller than his hand and hangs from his waist. The tinkling reassures him for he already senses death and the lapdog can't save him. He will die when he is four.  An idiot hunter, with white dog and carbine, still cannot decide whether the fact that he is king is a good or bad joke.  A peasant in some Naples back street (17th century). His hands that have foraged in everything now hold a manuscript and a compass because he's playing at being literate, pretending to be a visitor to the city. He's laughing. Behind the mask of laughter, his eyes miss nothing. He is thin as an elder tree. He has a question which is simultaneously an answer: who has the last laugh?  On the eve of the 20th century, an adolescent boy holds an open book in both his hands and gazes into the distance. He is sitting indoors at a table but is dressed to go out. Impossible to guess whether he's remembering or foreseeing. Between the pages of the book, he has slipped other pieces of paper. Notes? A letter? A poem? 'Your gadgetry/ makes me want to undress/ I'll feed the babe/ and naked beneath the sheet/ wait for your address.'  If you turn round to look back to where you've been, you'll see Aesop the story-teller standing in the middle of the gallery. Nobody's dupe, continually astonished and never surprised by his astonishment.  Overwhelmed by History (or its absurdly declared End), misled by the notion of Progress (which, nevertheless, exists), we tend to forget that nine-tenths of what we live has been lived during millennia before us, by others. The Spanish Portrait exhibition is a reminder of this.  It is a fact that no other national painting tradition could cover five centuries and find so many vividly contemporary portraits. I have no new explanation. Ortega y Gasset and Unamuno have several. Spanish portraiture is connected with a particular recognition of the centrality of pain and dignity, and this leads to examining life unflinchingly. You find exactly such a look in Velázquez's superb portrait of Luís de Góngora.  On the cover of the catalogue stands the Duchess of Alba (1797). Her eyes are bewildered. She points at an inscription scrawled in the sand at her feet. It reads: 'Sólo Goya [only Goya].'  'I can strip you as well as I can paint you,' he told her. 'That's where I have an edge over Velázquez. No mirrors. I advance on my stomach. Mix my colours with spunk.'  'Your colours, sir, are your business. It will be done from memory. You will paint me when you are alone. You will remember all the women you have known, all the women you have stripped - as you so eloquently put it - you will close your eyes and see them again, and then you will use all your effort, all your virility, all your speed, to recall what distinguishes every square millimetre of the body of the 13th Duchess of Alba from the body of any other woman now or hereafter.'  She is also pointing at her feet which, in their shoes, have the authority of two daggers in golden sheaths.
To walk among 100 beings, to sense 100 unique lives... a stunning exhibition of five centuries of Spanish portraits offers a quite remarkable experience