In dealing with architecture Canaletto was 'less to be trusted in its renderings of details than the rudest and most ignorant painter of the thirteenth century' ( Stones of Venice Works, 11.365). There are to be found 'physically impossible perspective, daubed mosaics and no colour' (quoted in Unrau, Ruskin and St. Mark's, p 213, from sketch and notebooks for The Seven Lamps of Architecture Princeton III 48), though it is unclear which picture(s) Ruskin had in mind. Unlike the paintings of Carpaccio and Bellini, the architecture is false in colour ( Works, 24.162).
In Ruskin 's Notes on the Louvre the painting of the La Salute ( Constable, revised Links, Giovanni Antonio Canal 1697-1768, No 169) is 'cold and utterly lifeless - truth is made contemptible' ( Works, 12.468), and with reference to the same picture, 'boats and water he could not paint at all' ( Works, 13.498), though that particular picture is now more commonly attributed to Marieschi.
Ruskin provided a visual summary of his argument in 1876 by 'painting against' Canaletto the watercolour of the Scuola di San Marco reproduced as the frontispiece of Volume 11 of the Library edition ( Works, 11.ii).
In words Ruskin summarises his objections at the end of Modern Painters V ( Works, 7.253 and Works, 7.254). The contemplative (and that might be translated as being associated with 'theoria') is directed principally to the observance of the powers of nature, and providing a record of the historical associations connected with landscape. Turner is the representative. The picturesque, of which Canaletto provides one example, is a 'degradation' of the contemplative, meant only to 'display the skill of the artist and his powers of composition'.