Notre Dame de Mantes is a sister to the Cathedral of Paris, "built at the same time, perhaps by the same architect, and reproducing its general dispositions, its mode of structure, and some of its details"; but the Cathedral of Paris has been greatly altered, so that its original arrangement is quite changed, while the church at Mantes remains practically as it was, when both were new, about the year 1200. As nearly as the dates can be guessed, the cathedral was finished, up to its vaulting, in 1170, and was soon afterwards imitated on a smaller scale at Mantes. The scheme seems to have been unsatisfactory because of defects in the lighting, for the whole system of fenestration had been changed at Paris before 1230, naturally at great cost, since the alterations, according to Viollet-le-Duc (articles "Cathedral" and "Rose," and allusions "Triforium"), left little except the ground-plan unchanged. To understand the Paris design of 1160-70, which was a long advance from the older plans, one must come to Mantes; and, reflecting that the great triumph of Chartres was its fenestration, which must have been designed immediately after 1195, one can understand how, in this triangle of churches only forty or fifty miles apart, the architects, watching each other's experiments, were influenced, almost from day to day, by the failures or successes which they saw The fenestration which the Paris architect planned in 1160-70, and repeated at Mantes, 1190-1200, was wholly abandoned, and a new system introduced, immediately after the success of Chartres in 1210. As they now stand, Mantes is the oldest. While conscientiously trying to keep as far away as we can from technique, about which we know nothing and should care if possible still less if only ignorance would help us to feel what we do not understand, still the conscience is happier if it gains a little conviction, founded on what it thinks a fact. Even theologians--even the great theologians of the thirteenth century--even Saint Thomas Aquinas himself--did not trust to faith alone, or assume the existence of God; and what Saint Thomas found necessary in philosophy may also be a sure source of consolation in the difficulties of art. The church at Mantes is a very early fact in Gothic art; indeed, it is one of the earliest; for our purposes it will serve as the very earliest of pure Gothic churches, after the Transition, and this we are told to study in its windows.