John Ruskin was a 19th-century critic. You sometimes hear him called an art critic but the truth is that he was a critic of everything.
I like his writing, though half of it is half-mad. I think he was a genius and also something of a failure as a human being.
The tragedy and greatness alike of Ruskin is that he was able to stay so completely within his own mind – he had a clearer perception than most people of what went on in there, of the richness of ideas of which his mind was capable – that he never learned to come out of his mind far enough to love people for their own sake: he loved them only as part of that beautiful cosmos in his mind.
When he met people in person he was disappointed by the disparity between his idea of them and reality; he really couldn't handle it. (This hubris killed his marriage.) He had the same problem with landscapes.
He was a walking experience of the tragedy inherent in Plato's theory of ideal forms, the idea that this world is just a shoddy imitation of a beautiful abstract. At the end of his life he retreated into his mind permanently.
I think of Ruskin as one of those elements toward the far right on the periodic table: so full of electrons, but not quite enough; made unstable by the sheer frustration of it.
You can see (or anyway I see) in Millais' famous portrait of him that he is surrounded by a talkative landscape, including the gneiss rock he loved, but he seems to be looking at nothing that actually lies before him.
The real reason is probably that although the painting was begun by that Scottish stream, the figure of Ruskin was filled in in Millais' studio. Ruskin was all about Truth in Art and No Compromises (he once forced Millais to return to a site to fill in a last few square inches of shrubbery) so it is perhaps telling that he allowed this artifice of being painted separately from the world around him.