A kind of self-portrait

The English portrait painter Joshua Reynolds was 24 in 1747 - still at the start of his career - when he painted his remarkable ''Boy Reading.'' Whoever the sitter was, this early portrait is much more than a ''likeness.''

It has an extraordinary introspection. It is self-revealing in a number of ways. It is distinctly Rembrandtesque. It is the earliest known instance of something that shows itself repeatedly in his art - his exceptional understanding of children. And also, like much good portraiture, it is a kind of self-portrait.

Ellis Waterhouse has written of Reynolds's upbringing in Devon, in a family of the ''lesser gentry,'' that ''the household was bookish, pious, and always short of money. The young Joshua benefited from the books....'' And he continued to benefit from books throughout his life - a very hardworking man, eager for knowledge and improvement of his mind. His friends were mainly literary men - Johnson, Boswell, Goldsmith, Burke, and the actor Garrick. Reynolds himself produced some notable writing about art, his ''Discourses,'' as the first president of England's Royal Academy.

From the age of 17 his apprenticeship as a painter had been in the hands of the portraitist Thomas Hudson. This teacher had fostered his respect for the old masters by giving him access to his collection of drawings and engravings. Learning from the old masters was just as important to Reynolds throughout his life as reading was. This desire took him to Italy from 1749 to 1753, particularly Rome, where he studied and gained a deathless admiration for such artists as Michelangelo, Raphael, and Correggio. As a painter, rather than a theorist (and the two sides of his character were not entirely consistent), he also learned much from the Venetian painters of the 16th century, notably that splendid exponent of the portrait, Titian.

But before he went to Italy he had already - as ''Boy Reading'' makes clear - developed an affection for Rembrandt which never left him. In fact, it was so strong that in about 1773 he was to produce an imposing self-portrait in which he presented himself as Rembrandt, unmistakably. For ''Boy Reading,'' Rembrandt's portrait of his son Titus (now in Rotterdam) is the main inspiration. It is an interesting example of the way admiration for another artist can be the stimulus of an original idea: Indeed, Reynolds's frequent quotation (the unkind might dismiss it as ''borrowing'') from earlier masters is generally of this order. Perhaps this characteristic is only a rather noticeable instance of what is true of virtually all artists. Admiration is at least as strong an incentive to fresh endeavor as the more suspect notions of ''special genius'' or of ''artist as creator.''

The Rembrandt and the Reynolds paintings have intriguing differences for all their similarity. Titus seems almost bored with his reading matter, like a child with homework who would rather be playing ball. His eyes gaze distractedly into space. Reynolds's boy - himself in retrospect? - holds his book eagerly, and, if his eyes do wander momentarily from the page, his mind is still firmly on the book. The light playing subtly over his features - one of Reynolds's wonderful skills as a painter - seems to come almost symbolically from the book, as well as from a physical source.

All told, this is a picture of solemn, absorbing boyhood application, painted by someone who knows.m