Shaw's contempt for science was indefensible. Like Tolstoy, he couldn't believe in the importance of anything he didn't know. He was passionate against vivisection. I think the reason was, not any sympathy for animals, but a disbelief in the scientific knowledge which vivisection is held to provide. His vegetarianism also, I think, was not due to humanitarian motives, but rather to his ascetic impulses, to which he gave full expression in the last act of Methuselah.
Source: Bertrand Russell: Bernard Shaw, 1953