Ruskin played a significant part in the nineteenth century re-appraisal of the canon of Italian art, both painting and sculpture.
Reynolds assumed that Italy had produced the greatest painters, and that among Italian painters Raphael and Michelangelo were in competition for the first place. Ruskin in Modern Painters I assumes that his readers will accept the established view of the greatness of Italian painting, and of the 'classical' Roman tradition of sculpture which influenced the work of Canova, which derived from from the Apollo Belvedere, the Torso of the Vatican, the Venus de' Medici and the Laocoon, and which was challenged by increasing knowledge of the Friezes of the Parthenon.
A second assumption, following Vasari, was that there had been an organic process of growth and decay in Italian painting. Ruskin is convinced of the reality of the decline of Italian painting, though his dates for the start of the process, and the painters he would cite as examples of decay are not always those which most of his readers would have accepted. Ruskin came to challenge ideas of progress in Italian painting with his recognition of the greatness of Giotto, and the assertion at Works, 24.40 that Cimabue is both the first and the greatest of the Tuscan painters. Similarly Giovanni Bellini and Angelico are valued more highly than Domenichino and Dolci. (The wider context for these changes of judgement is discussed in Haskell, Rediscoveries in Art and Haskell and Penny, Taste and the Antique.)
A third assumption was that the reputation of Italian painting had produced a lack of discrimination in response to Italian painting in general, and to individual works attributed to great masters. Ruskin's aim is to provide a set of clear and consistent principles which might lead people to 'appreciate worthily', both in Modern Painters I and, for example, at Works, 18.435 and Works, 18.436, and Works, 20.25. The corollary of that is the strenuous depreciation of those Italian painters who had, in Ruskin's view, been wrongly praised for their truth to nature. Canaletto and Salvator Rosa are singled out along with Gaspard Dughet/Poussin and Claude, who, though not Italian by birth, were in their work members of the Italian school (see definition of the Italian school).
One outline of Ruskin's judgements of the Italian school made in 1845, its sub-divisions and the hierarchies of value within those sub-divisions, is to be found at Works, 4.xxxiv. There is a later analysis in 1871 in The Relation between Michael Angelo and Tintoret, starting at Works, 22.77. This analysis is based on the distinction between Florentine and Venetian schools and is summarised:
Her iridescence of dying statesmanship-- her magnificence of hollow piety, -- were represented in the arts of Venice and Florence by two mighty men on either side -- Titian and Tintoret, -- Michael Angelo and Raphael. Of the calm and brave statesmanship, the modest and faithful religion, which had been her strength, I am content to name one chief representative artist at Venice, -- John Bellini. ( Works, 22.81)