So far the Popes had endeavoured to supersede the bishops in the issue of indulgences by entering into rivalry with them. But the power was used by the bishops in such detailed ways as perhaps seriously to interfere with the offerings which should reach the Papacy or be applied to important projects. Innocent III, therefore, at the great Lateran Council limited the episcopal power to the grant of an indulgence for one year at the consecration of a church and for forty days at the anniversary. Unfortunately this did not mean the suppression of trifling reasons for the multiplication of indulgence. The whole system was a convenient method of adding to the revenues of Rome, and no occasion seemed too small for the exercise of the papal power of dispensation. Urban IV granted an indulgence to all who should listen to the same sermon as the King of France. The Crusades were the great occasion and excuse for the development of this system, and it certainly reached its nadir when Gregory IX showed himself ready in return for a pecuniary penance to absolve men from the vows which they had perhaps been unwillingly forced to take by his own agents for going on crusade. Equally disgraceful was the establishment of the year of Jubilee in 1300 by Boniface VIII, when plenary indulgence of the most comprehensive kind was offered to all who within the year should in the proper spirit visit the tombs of St. Peter and St. Paul at Rome. [Sidenote: Treasury of merits.] But how came the Pope to be in possession of this power of remitting the penalties for sin? The schoolmen of the thirteenth century supply the answer. Alexander of Hales and Albert the Great invented the theory and Thomas Aquinas completed it. According to their teaching, the saints, by their works of penance and by their unmerited sufferings patiently borne, have done in this world more than was necessary for their own salvation. These superabundant merits, together with those of Christ, which are infinite, are far more than enough to fulfil all the penalties due for their evil deeds from the living. The idea of unity in the mystical body enables the shortcomings of one man to be atoned for by the merits of another. The superabundant merits of the saints are a treasury for use by the whole Church, and are distributed by the head of the Church, that is, the Pope. Furthermore, to St. Thomas is due the idea that the contents of this treasury were equally available for the benefit of souls in purgatory, for whom the Church was already accustomed to make intercession. [Sidenote: Canonisation of saints.]