[248] fundamental principle. In fact, we may say it is accepted as a test of enlightenment, and the man in the street is forward in acknowledging that countries like Russia and Spain, where opinion is more or less fettered, must on that account be considered less civilized than their neighbours. All intellectual people who count take it for granted that there is no subject in heaven or earth which ought not to be investigated without any deference or reference to theological assumptions. No man of science has any fear of publishing his researches, whatever consequences they may involve for current beliefs. Criticism of religious doctrines and of political and social institutions is free. Hopeful people may feel confident that the victory is permanent; that intellectual freedom is now assured to mankind as a possession for ever; that the future will see the collapse of those forces which still work against it and its gradual diffusion in the more backward parts of the earth. Yet history may suggest that this prospect is not assured. Can we be certain that there may not come a great set- back? For freedom of discussion and speculation was, as we saw, fully realized in the Greek and Roman world, and then an unforeseen force, in the shape of Christianity, came in and laid chains upon the human mind and [249] suppressed freedom and imposed upon man a weary struggle to recover the freedom which he had lost. Is it not conceivable that something of the same kind may occur again? that some new force, emerging from the unknown, may surprise the world and cause a similar set-back? The possibility cannot be denied, but there are some considerations which render it improbable (apart from a catastrophe sweeping away European culture). There are certain radical differences between the intellectual situation now and in antiquity. The facts known to the Greeks about the nature of the physical universe were few. Much that was taught was not proved. Compare what they knew and what we know about astronomy and geography—to take the two branches in which (besides mathematics) they made most progress. When there were so few demonstrated facts to work upon, there was the widest room for speculation. Now to suppress a number of rival theories in favour of one is a very different thing from suppressing whole systems of established facts. If one school of astronomers holds that the earth goes round the sun, another that the sun goes round the earth, but neither is able to demonstrate its proposition, it is easy for an authority, which has coercive power,