Thomas Couture, Paysage: Entretiens d’atelier (Paris : Typ. de L. Guérin, 1867), p. 139 (my translation):
As I have already said, and it is something one cannot say too often, landscapes are an artistic degradation; the category never existed in the golden age of painting, and yet Raphael, Correggio, Titian, Veronese, Rubens, Rembrandt, Velasquez (and others still) were the greatest landscape painters in the world.
Poussin should not be considered a landscape painter, but a philosophical painter, and that is what makes his style inimitable.
Claude Lorrain is not a landscape painter, but a painter of light and boundlessness, and that is what makes his style inimitable.
And so the landscape artist’s role begins with a certain mental inferiority.
I disagree with Couture, but enjoy reading his diatribes.