Perhaps the most notable and thorough effort to provide a scientifically plausible account of invisible spirit worlds within a Christian context was made by the distinguished Scottish physicists Balfour Stewart and Peter Guthrie Tait in their book The Unseen Universe (1875). Although Stewart became president of the Society for Psychical Research during the 1880s, both men were sceptics of spiritualism, seeing in it nothing more than evidence of human suggestibility. Tait attacked spiritualists at the British Association meeting of 1871, bracketing them alongside “Circle-squarers, Perpetual-motionists [and] Believers that the earth is flat”. Yet he and Stewart were eager to understand how the “invisible order of things” that the Bible seemed to demand — the existence of immortal souls — might be consistent with the laws of physics. They aimed to refute Tyndall’s attack on religion in his address to the British Association in Belfast in 1874, in which he asserted that religion should not be permitted to “intrude on the region of knowledge, over which it holds no command”. On the contrary, Stewart and Tait insisted, science and religion were fully compatible. Yet their version of Christianity, on the evidence of The Unseen Universe, was starkly materialistic: they fit within a long tradition of both advocates and opponents of religion who insist on making it a set of beliefs about the physical world that may either be rationalized or disproved.