Canaletto painted canals, and they are examples of calm water. Ruskin suggests that with calm water, as opposed to sea, facts are ascertainable and demonstrable. He seeks to prove that Canaletto's treatment of water in Venice is false by judging it against the 'most common and general optical laws which are to be taken into consideration in the painting of water' on pages MP I:327-33. Ruskin's response to C.R. Leslie's criticism of this section seems unhelpful and even petulant in its assertion of the priority of general principles and an irrrelevant observation.