One of the only times the National Gallery has organized an appeal to save a painting for the nation was in 1971 for Titian`s Death of Actaeon, 1562. It was for some a miracle along the lines of Dunkirk, and our teacher put us on a train from Shrewsbury to see it. Five years later, while at Chelsea, I got to know the painting better, and since then have had the same photograph of it on a succession of studio walls. Although it is not in the exhibition in Venice it has all the qualities of the greatest paintings by Titian. 

At Chelsea I was trying to find a language whereby I could paint figures in a spontaneous way. I lacked confidence with oil paint, and through drawing a lot, and drawing with paint, I hoped to free up to become as fluent with figures as I had been with landscape. I did not really envy the freedoms of the current abstraction: I could not find myself involved in it and wanted to be more certain about the connection between myself and what I painted.

Not all paintings on display in the Ducal Palace have the same 'absolute Titian' quality, and there are such varying degrees of authenticity that one has to think hard about where Titian is in each case. Is he completely present, as in every square inch of The Flaying of Marsyas, 1575-6? In other cases, did he paint the painting, and has it been retouched? Is there an oil sketch, a beginning, from his hand under the surface?. Or is his presence merely the assistant's employee who will be back a week on Tuesday – as one feels with Penitent Magdalene from Naples? For me, everything important in Titian is in the language of his touch, which has to be there. 

This gets clearer in the late works. In the early works his language encompasses other influences, as I would expect – particularly Giorgione, who is perhaps unwittingly present in the form of the disputed Christ Carrying the Cross, 1508. Giorgione's handling of oil paint had a life in it that had never been seen before. He had dispensed with working from meticulously planned compositions, conceived in detail before the painting was started. Painting was no longer the last stage of the process. 

'Christ carrying the Cross', Titian, oil, 1508, 70 x 100 cm

The presentation of the Venice exhibition is excellent and includes infra-red revelations of the early stages of many works, which evidence frequent reworking away from the original position of the figures. The paintings can be seen to have grown organically, conception and process becoming one and the same. Vasari considered this to be a kind of naive short-cut and a lack of planning. It was the antithesis of the Disegno technique employed by his beloved 'mortal God', Raphael. The paint quality resulting from Titian's revisions, and building up from his very bold beginnings, is precisely what makes these works so rewarding for us. It did occur to me how much I would rather see the early stages completely revealed – the real spirit of Titian, as opposed to the rather patchy last layer which , as in Annunciation, 1542, FORM San Rocco, is a combination of Titian, an assistant, general wear and tear, changes in proportion, repainting, and restoration. I am glad to see it survive, but it reveals little about Titian. His bold original stages tantalizingly exist beneath. Even during his lifetime, the bravura of Titian's rapid and spontaneous drawing with thick paint was considered fascinating and widely admired. I think we falsely credit ourselves today with the modern appreciation and comprehension of painterliness.

To look in detail into Titian's work evokes a degree of misgiving. I feel a reluctance to examine what I have felt instinctively till now. Is it possible to examine the exact mechanism by which the transition of thought in Paul III's eyes is also transmitted by his fingers? And how Titian had the courage to paint it? There seems to be a healthy unselfconsciousness in the artist's relationship with his sitter. We have lost this quality. It was not an issue to paint a portrait of a Pope. It was first of all a portrait, which was needed. There was an acceptance of the value of doing it, and a collective intellect to assess it. It was how one did it that mattered, and, because it mattered so much to those concerned, the artist did it in the most interesting way he could. He could then paint Bacchus drinking himself into a stupor and would not expect today's critical response: 'the new work's really changed: he's imitating Roman art and no longer believes in Jesus'. 

I envy this freedom of mind: painting was comprehended in a broader way and connected with other disciplines. It may seem I am the ungrateful inheritor of a century of 'barrier breaking' but it sometimes feels as though the baby was lost with the bath water.

It's still how one does it that matters; but where is our subject matter? Nothing unselfconscious comes on a plate for us today. My own painting, Ochre Mine, 1989, is about looking for that subject – metaphorically in the earth's pigment itself. 

Pope Paul III's portrait (1543) works like a spontaneous ink drawing, but with a resonance only possible in oil paint. It is from the middle of Titian's long career. It is virtually monochromatic at a glance, which means that, as with the intense stare, I get a particular sensation from the predominant colour, which itself evokes a mood. In this case it's an old, but grand, blackening crimson, sumptuous and gold-stitched, which encases the old man. His head emerges like a tortoise's, his left hand passive but his right hand eloquent. Here the fingers are definitely 'rendering unto Caesar and fan out to echo the striations on the Papal purse. The whole image is charged with an electric presence, the simple and strong light bringing the Pope towards the viewer out of the dark background. It has the feeling of a first impression, yet it contains an entire human presence, sensed in the nerves. This is not achieved by slavishly copying nature or by virtuosity. Titian is tapping his own experience of life as he paints. He has adapted visual reality to enhance where he has need to enhance his chances of making this happen. For instance, the cloak comes down very low to an undulating edge that bisects the canvas two thirds down.

We know Judith's fingers are sunk into the severed head's hair, but, by partially obscuring them, what a carnal shape the painter has made. They become claws, yet what I love in Titian (and what I suspect gives him his power) is that he does not overstate it: they still look like fingers. However, the main interest for me in this painting is the way the heads are treated and the way in which a variety of treatments are quite freely applied: they seem to be employed to create contrasts of expression.

'Martyrdom of St. Lawrence', Titian, oil, 1559, 493 x 277 cm

Saint Lawrence (Martyrdom of Saint Lawrence, 1559), on the other hand, has hope. He is loved by his God and, despite his agonizing death, there is space, and God is present in that space. If Saint Lawrence had been portrayed like Tityus, it would have been completely unacceptable to the Church; perhaps it would even appeared to be the work of an atheist. But again Titian is very direct, taking the instrument of martyrdom as his basic visual theme; the flames seen through the gridiron are symbolically omnipresent throughout the painting. Firstly, we have the armour rivets reflecting the fire in the form of a grid. Saint Lawrence's gesture takes our eye to the blazing torches which are baskets of fire echoing the gridiron itself, and then to the light behind the window which also shares the perspective of the foreground, and eventually the to God's light, seen through a hole in the clouds. Like a varied, recurring musical theme, we have a visual theme which simultaneously describes the presence of God and the merciless fire. Does not this, at a profound level, imply God's presence in suffering, so that the fabric of this painting's composition is saturated in visual poetry? 

Titian seems to be able to locate the exact psychological quality which is at the heart of whatever he depicts. He finds the appropriate sense of tension in a very specific way, and his process has been evolved to make this happen. I admire the way his openness of mind allows a painting to take a particular direction, unforeseen at the start but maintained in the unconscious while working, trusting that all will be sensed when the work is finished. In my painting Heart of a Glacier, 1989, I have tried to work in a similar way. I used the mental sensation of a glacier: the coldness and lethargy of its slow progress through time; the idea of its layers and what is has accumulated as it creeps along. And with that going around my mind I just started to paint figures, sometimes from models, sometimes not. Even near the end, I do not know if any is going to come of it. All I know is that I have put a lot into it which can be adapted or reduced if necessary. I can have a much more enriched relationship with a painting if I am not restricted by preconceived aims. 

'Tityus', Titian, oil, 1549, 253 x 217 cm

More important, and something which art historians seldom consider, is Titian's motive for painting a picture. I cannot believe that there is no connection between Marsyas, punished for losing a musical contest with the Gods, a contest of creation, and Titian himself, an old man within five years of meeting his maker after a long lifetime of painting. The Greco/Roman myths act as metaphors for the mysteries of our own existence. Marsyas is a theme that was taken up by several other artists of the day, as in Guido Reni's version, in a rather melodramatic way with Marsyas screaming in agony. Titian's Marsyas is too terrified, anticipating his fate. Perhaps he moans, but if you look at the painting upside down his eyes reach out to you and say, "this isn't a nightmare of powerlessness, is it? It's really happening to me". While looking at the painting inverted its worth noticing the extraordinary elongation of Marsyas's body. It strides into another reality altogether. As with Saint Lawrence, Titian uses the method of punishment as the theme for describing the other key parts in the image. It's really a composition of skin hung against a richly atmospheric and earthy background. It's so simple in this respect. Marsyas's torso is the largest skin formed and is pinned visually by the navel, which very nearly marks the centre of the canvas. (I am not sure how significant that is as the canvas has been cropped). The shape of its torso is stretched but is quite full like a fruit, and as a form is contrasted with the linear quality of four or five arm forms that surround it. These arms are all bent at the elbow and their acuteness is something like a rib in the canvas torn downwards. For me, they create precisely the sensation of the flaying itself – the peeling of skin. Their relationship to each other created a rhythmic quality as though as it is all done to music, and this is especially sinister.

'La pieta', Titian, oil, 1576, 378 x 347 cm

'Titian' The Death of Actaeon, oil on canvas, 178 x 198 

I loved my black and white photograph of Actaeon because it showed the painting without the frame. I could see the edges, and it looked like a painting in the physical sense that painters know them. Titian's presence was for me excitingly tangible in the painting, and the beautifully sustained composition and subliminal symbolism in a very painterly surface achieved a tension and a freshness that had eluded my own efforts. By symbolism I mean that Titian contrived to repeat themes in different ways in the same painting. In this case, the ivy winds slowly but fatally up the tree on the right, very much as the dogs pursue and overpower their master. 

 Once Actaeon started to be changed into a deer, it was natural that he become the victim of his own hunting dogs; a cruel irony of nature, but the choking ivy is doing the same. These late works are most interesting to me because Titian's mastery of the medium allows him to reveal an internalized reality which is given a voice simultaneously by the painterly and the pictorial content. To choose the exact moment when Actaeon is metamorphosing emphasizes the lively state of flux in which we see this painting, and in a way underlines the highly mobile way it has been painted. He uses the medium so fully – an organic range of oil paint in all its states from thick and dry to expansive stains and glazes. It's a technique that articulates while it expresses, the life-giving and spontaneous quality never drowning the nuance and the subtlety. 

'Salome', Titian, 1511

Perhaps I should not use an example with disputed authorship, but Christ Carrying the Cross in the exhibition as a Titian and it's worth comparing it with Old Woman, 1508, which is definitely Giorgione. It seems to me that the Christ has been worked almost like a fresco – the light and dark areas being easily separated and the definition of hands and faces contained within strong and established shapes. In Old Woman, on the other hand, the handling reflects rather the subtle qualities of oil paint, achieving a profound and insightful honesty. Both paintings depict 'exquisite pauses in time' as Walter Pater expressed it; but Giorgione appears to deepening a self- reflective stance, while we mentally expand Titian's condensed view of humanity. The way his figures occupy a space parallel to the picture plane has an emphatic quality that fires the very late works 60 years on. 

'Ochre Mine' Simon Edmondson, 1989, oil on canvas, 228 x 233

In the best cases, this aware and intimate contact with the surface of the painting (something that cannot exist in any preparatory form) is sustained until completion, and then the content, the props and presences are completely implanted into the composition and the tiniest details are perceived as cells of the larger structures, rather than as finishing touches or as appliqué. I think this is also why when something is wrong in one of these paintings it shows. The pediment at the top of La Pieta, 1576, for instance, is absolutely not how Titian would have liked it. He might have had a pediment, but it would not have competed with the figures but have risen into a softening shadow – I am certain. 

'Flaying of Marsyas', Titian, oil, 1575-76, 212 x 207 cm

Compare Pope Pail's right hand with Judith's left (Judith, 1567-8).  The severed head is not so gory, but it's bruised and portrayed as an object. Its presence is achieved by its being so much larger than the other heads in the painting, and by the fact that it looks so heavy. I had a pig's head in my studio for a while, and to lift it gave me a resounding and ironic sense of mortality. Titian contrasts all this with a coquettish Judith, her skin fresh and unblemished: butter wouldn't melt in her mouth. But her arm is less feminine, and her hands are ruthless. It's worth comparing this treatment with an early interpretation of the theme. In Salome 1511, I find a much more tranquil and idealized image. The heads are treated purely with beautiful gradations and are calm, mysterious grace that pervades totally. It's in a way ideal that Salome should look a little remorseful. Judith's contrasting callousness indicates Titian's later tendency to use an opportunity like this to portray a darker and more cynical view of humanity. The earlier work shows a different motive. He is establishing himself as a master of a radical new style; but later in life he shows us an inner reality. The colour has become entirely fragmented, is used very directly, and not to create a sense of equilibrium as it does so beautifully in Salome. It's sad that this exhibition did not include more examples of the female portraits: Salome is not here, nor is La Bella, nor Sacred and Profane Love.

A quality that interests me a lot depends on Titian's use of the picture plane; even in the early Christ Carrying the Cross, the figures have a strong dialog with it. In the very late works, the picture plane has a magnetic tension with the main figures, which are often conceived in a parallel space; we are only aloud to see through them into the distance beyond, as in Actaeon or Marsyas. For me, it's very true to the nature of painting that its two dimensions should be proclaimed and the illusions of three dimensions should be played like a fish – retrieved as much as allowed. Tityus, 1549, is a clear example of this. The painter has emphasised that Tityus is a giant, and equally that his fate of perpetual torture is inescapable. Convert this inescapable theme into the language of Titian's painting and we have a figure whose limbs are all pulling in opposite direction. At the same time they are chained within a narrow, illusionary space just behind the picture plane. The equal scale of the chains, stress this feeling of physical and illusionistic restriction within the confines of the canvas. The foreshortening is minimized; there is no relief in the distance. Nothing lets in a sense of hope. 

'Heart of a glacier', Simon Edmondson, 1989

In Titian, I also like the sense that the work is going on, and in the very late works the process seems almost endless. Art historians often theorise about how certain works remain unfinished. I do not think it's the most useful way to look at this phenomenon: Titian is undertaking something that does not necessarily include an idea of finishing. 

'Judith', Titian, oil, 1576-68, 112 x 94 cm

It is painted so directly that I am always surprised by how much subliminal colour there is. It's conceived mainly in tones, but the light areas are divided into either the warm skin tones or the cold whites from the sky, dog's hair and some garments; these latter details are subservient to the naked flesh, although with other details they appear widely across the canvas. So too do the characteristic crimsons, blues, and greens of the earlier works which are laid in as if they were jewels in the grass or partially hidden berries, so that they enrich the surface without detracting from the exquisite organization of tones.

This is the quintessential Titian in the exhibition, and it's an inspiration in the way the complex figurative elements are brought to create a single and unforgettable sensation. It's here that I find Titian himself, the different and conflicting parts that make up the complete person, revealing himself to us as the layers come away. 

'Titian: Prince of Painters', until 7 October, Ducal Palace, Venice; 28 October.27 January 1991, National Gallery, Washington D.C.