MANY members of the clergy of my acquaintance share a common anxiety about the mail. Unprovoked, and often abusive, letters turn up unexpectedly, while anonymised claims get lodged randomly with archdeacons. Not all are maliciously intended, but, now that social media are becoming all-pervasive, a chance remark or a misunderstanding can be widely circulated mischievously.

I can sympathise with the Vicar at Englefield Green receiving the following unwarranted tirade from somebody he was trying to assist.

"Especially if I should have to answer for it I feel very sorry that your ideas and Mine . . . differ so much as to have made you angry. . . If I am wrong I am in good company. . . But you ought to know what is Grand is necessarily obscure to weak men. That which can be made explicit to the idiot is not worth my care," etc., etc.

There you have it.

In fact, the correspondence is dated 23 August 1799, but, as I read it, a shiver ran down my spine, as it felt all too real. William Blake had been put in touch with the Revd Dr John Trusler as a possible patron by George Cumberland, to whom he had taught etching techniques, and fell out with him spectacularly about what he defined as "Moral Painting".

Blake, it turns out, was good at biting the hands that fed him.

He was also a man of firm opinion, as his 1798 copy of an influential edition of lectures given by the founder president of the Royal Academy (The Works of Sir Joshua Reynolds) on display in the current exhibition at the Ashmolean Museum in Oxford shows; on the title page, the impassioned artist has scribbled, "This man was Hired to Depress Art This is the opinion of Will Blake my proofs of this Opinion are given in the following Notes."

As indeed they are. In one piece of doggerel in his 1808 Notebook, he dismisses an Old Master whom Reynolds had celebrated:

You must agree that Rubens was a fool
And yet you make him master of your school
And give more money for his Slobberings
Than you will give for Raphael's finest things.

Yet, as this latest exhibition by Michael Philips, the doyen of Blake's world, makes clear, Blake was well schooled among the antiquaries, and his early work reflects a deep knowledge and understanding of the Old Masters and a very traditional apprenticeship that followed an ambitious programme of drawing as a child.

At the outset, Blake, a hosier's son from Soho, was able to copy casts of classical sculptures which his father bought for that express purpose. Even before becoming a teenager, he was a regular visitor to the London houses of the nobility to see fine paintings. This sort of "Open House" arrangement had been promoted for interested scholars and artists alike from 1740 onwards, alongside the founding of the Society of Arts.

Blake made a habit of visiting Christie's and other auction houses, sketchpad in hand. In all the years I have walked through the showrooms in Kensington and Mayfair, I have not spotted any aspiring artists.

After four years at drawing school, he served seven years with James Basire, from the age of 14 in 1772. The first galleries are full of drawings of the medieval tomb effigies in Westminster Abbey for Richard Gough's 1796 ground-breaking work, Sepulchral Monuments in Great Britain, which preoccupied Basire at the time.

Thanks to the generosity of the Bodleian Library, a range of medieval illuminated manuscripts that, it is suggested, informed Blake's own decorative styles, distinctive curlicues in the margins causing havoc with the text, is on show.

He was also, as were all British artists who had never had the opportunity to cross the Channel, indebted to the engravings of Old Master paintings. For a publisher who asked him in 1780 to illustrate The Protestant's Family Bible he copied the Old Testament scenes that Raphael frescoed for the Vatican Loggias from his own edition of Raphael's Bible which he had bought as an apprentice in 1773.

Sisto Badalocchio (born 1585) and Giovanni Lanfranco (1582-1647) had etched drawings of the Raphael frescoes that the great Bolognese artist Annibale Carracci (1560-1609) had made when he moved to Rome. The chain of transmission may strike us as odd in our age of virtual (and immediate) communication, but, in the era before photography, artists relied upon copying, itself made much easier with the invention of printing.

From Raphael to Carracci to Lanfranco and Badalocchio to the front room of 13 Hercules Buildings, Hercules Road, Lambeth, may seem a circuitous route for an exhibition to take. But it was across the Thames that Blake moved, with his wife, and set up his studio ten years later, in January 1791. The curators have recreated the studio, following floor plans that were discovered in 2004.

The house itself was demolished in 1918, but how poky the cramped printing room (13 ft 7 ½ ins. × 12 ft) looks to us when we recall the scale of work that Blake printed here. He was able to take down and reassemble the five-foot-high flat rolling press, a job that would have required the assistance of a more than competent craftsman. The even smaller back ground-floor room served as his painting atelier.

His final residence in the 1820s until his death in 1827, at 3 Fountain Court, in the Strand, was just two rooms on the first floor of a house in which his wife, Catherine, was expected to cook in the bedroom.

For visitors of an artistic bent, this exhibition richly provides for a history of printing, and shows how well Blake managed his craft and developed his art. These include techniques for printing in more than one colour, and his innovation of relief printing, in which he combined the two very different processes required by printing text and providing illustrations. The dizzying complications of this made me even more amazed by his transferring reams of text in his inimitable hand.

The range of exhibits includes a pencil portrait of him, around 1785, by Catherine; his publication of Songs of Innocence and of Experience (1789); and the familiar images from the 1794 Europe a Prophecy, and busts of Milton and of Dante from the series of Heads of the Poets (c.1800-05), commissioned by William Hayley for his house in the seaside village of Felpham, Sussex.

Observation and carefully detailed drawing delineate all his works to the end, and the final gallery room takes Blake's later career into the lifetime of his pupils and those who sat at his feet: "the Ancients", as Samuel Palmer, George Richmond, and Edward Calvert chose to be known.

This ambitious and scholarly exhibition is complemented across the city centre with Jeremy Deller's display of works by William Morris and Andy Warhol, "Love is Enough", at Modern Art Oxford (to 8 March), a seriously playful and equally challenging exposition of artists one might think one knew already too well.

"William Blake: Apprentice and Master" is at the Ashmolean Museum, Beaumont Street, Oxford, until 1 March. Phone 01865 278000.

www.ashmolean.org