The price of pricele$$

Yes, £100m is a huge amount of money, but these two Titian paintings have proved their worth for 450 years

SO THE Duke of Sutherland is selling his two great Titians, Diana and Actaeon and Diana and Callisto. They have been in the National Gallery of Scotland for more than 60 years, but they were always his or his predecessors' to sell and now we finally face the moment we have been dreading. The National Galleries of Scotland and the National Gallery in London have joined together to try to purchase them. The asking price is £50 million for …Actaeon now and another £50 million for …Callisto in four years' time. It is a huge sum, but how do you put a price on what is truly priceless? There have been appeals to save paintings for the nation often enough, but none has ever mattered so much. These two pictures are quite simply in a league of their own. The story of modern art begins with them. Nor do I say that simply on my own authority – all the greatest painters who knew them, or who even only knew of them, paid homage to these two paintings, and not only those close in time to Titian.

Their potency has endured undimmed throughout the four-and-a-half centuries of their existence. Cézanne's ambition to paint a great composition of bathers, an ambition that he saw as central to his whole achievement, was inspired directly by these pictures. Even when he seems most radically modern in his Bathers series, he is only following where Titian led. In his Demoiselles d'Avignon, the very cornerstone of Modernism, Picasso pays homage to Titian's Diana and Actaeon and in our own time, these two great pictures have continued to inspire painters such as John Bellany and Steven Campbell.

So what is about the two Titian works that has earned such respect? They were painted for Philip II of Spain. Titian took a long time, but when he finally dispatched them, he humbly suggested the king would realise they were worth the wait. Titian had already painted a number of "poesie" for his royal patron – pictures with erotic subjects from the stories of Ovid – but he clearly saw these two as exceptional in his body of work. This was perhaps in part because when he embarked on them he had recently visited Rome where he met Michelangelo and saw his latest work, The Last Judgment. The Diana pictures are his response. We are eavesdropping on a conversation between giants.

There are visual links to confirm this, especially between Diana in …Actaeon and Michelangelo's terrifying Christ in The Last Judgment. It is not only her gesture that echoes his, it is also its meaning – both raise their arms to condemn transgressors.

These pictures are also unique in Titian's work in having Diana as protagonist and this is crucial to their meaning. She was the chaste and inviolable goddess of the moon; therefore the subject here is not amorous dalliance among the gods. On the contrary, it is Diana's implacable wrath. Actaeon and Callisto have innocently transgressed her law. Actaeon accidentally stumbled upon her naked and for his unwitting crime he was turned into a stag and torn to pieces by his own dogs. His grisly fate is hinted at by a stag's head on a stone column in the picture. His startled expression testifies to his innocence, but Diana is indifferent and condemns him with a gesture the power of which arcs like lightning along the jagged line formed by her companions. To reinforce this effect, the logical recession is denied: the arc of figures seems to be in one plane, though it actually recedes. To hold her likewise into this plane, Diana's figure is also strangely drawn, focusing all her power along the line of her fierce glance and in the gesture of her raised arm.

Callisto sinned against Diana's law of chastity, but would hardly have been condemned in any ordinary court. Jupiter himself seduced her and to approach the unsuspecting girl, he disguised himself as her mistress, Diana. But again Diana knows no mercy. She points at Callisto as her former friends strip her of her clothes and even her shoes, exposing her nakedness and with it her pregnancy. If Diana and Actaeon are joined by the goddess's gesture, here a diagonal break in the composition radically separates her from Callisto, emphasising her imminent exile and isolation. Without Diana's protection, Juno, Jupiter's jealous queen, turned Callisto into a bear, to be hunted by the son she bore. He was unwittingly about to kill his own mother when Jupiter intervened to turn her into a constellation: the Great Bear.

The two pictures, conceived as a pair, are closely linked visually. In both, Diana's divine wrath is in contrast to landscapes of exquisite beauty, but nature is also indifferent to any merely human presence. There is architecture, but it is fragmentary and ruinous, its verticals and horizontals are tilted and unstable. Titian's technique is direct and experimental; the paint surface is broken, defying all the conventions of finish. In a way that was quite new at the time and which shaped the art of painting thereafter, Titian is physically present to us by the vivid traces of his moving hand. He has abandoned the convention of artistic distance assumed by his predecessors and with it the luminous clarity and stability of High Renaissance art in favour of something that is at once immediate, full of energy and profoundly uncertain.

Extending that radical shift, in the figures, the classical ideal of beauty is quite forgotten. This is apparent above all in the pathos of poor Callisto's defenceless nakedness. Her terror is palpable. Shakespeare, Titian's younger contemporary, is the only possible comparison and in Shakespeare it is only King Lear that combines such pathos, truth and grandeur. As Lear in the storm recognises Tom's nakedness and says to him, "unaccommodated man is no more but such a poor bare, forked animal as thou art," so, as Callisto is stripped of her clothes and her privacy, we see how helpless her pitiful humanity is against Diana's divine and arbitrary wrath.

These pictures are a matrix, not because of their beauty alone, nor even their extraordinary originality, but for what together with their subject matter these things add up to. We see here for the first time in art the collision of our understanding of the glory of the world with our inescapable recognition of its inexorable, arbitrary indifference to our individual fates – nor is there any hint of the Christian message to veil that stark truth. These pictures are truly modern. In the 16th century the tectonic plates of human understanding shifted. The aftershocks of that earthquake still shake the ground beneath us. It was a unique moment. In response to it, Titian, Shakespeare and a very few others articulated a wholly new vision of the world and our place in it but, because they did so in response to a unique moment in history, they remain inimitable. We can neither replicate their vision, nor replace it. We can only wonder at it and hope to learn from it.

These two pictures have changed hands only twice. In the early 18th century they were gifted by Philip V of Spain to the Duke of Orleans, Regent of France. Sold during the French Revolution, with the rest of the Orleans collection, they were bought by the Duke of Bridgewater. They then passed by descent to the Dukes of Sutherland. They have been in Britain for almost half their existence and longer than they were in either France or Spain. What is more, they have been continuously available to the public since they came to Scotland in 1945 and were also readily accessible in London for much of the time before that. They are truly part of our heritage, part of the imaginative capital that is the vital endowment for our future. It is our duty to our successors to find the means to keep them.