From the port of Barcelona the Spanish khalifs had carried on an enormous commerce, and they with their coadjutors--Jewish merchants--had adopted or originated many commercial inventions, which, with matters of pure science, they had transmitted to the trading communities of Europe. The art of book-keeping by double entry was thus brought into Upper Italy. The different kinds of insurance were adopted, though strenuously resisted by the clergy. They opposed fire and marine insurance, on the ground that it was a tempting of Providence. Life insurance was regarded as an act of interference with the consequences of God's will. Houses for lending money on interest, that is, banking establishments, were bitterly denounced, and especially was indignation excited against the taking of high rates of interests, which was stigmatized as usury--a feeling existing in some backward communities up to the present day. Bills of exchange in the present form were adopted, the office of the public notary established, and protests for dishonored obligations resorted to. Indeed, it may be said, with but little exaggeration, that the commercial machinery now used was thus introduced. (Draper's "Conflict between Religion and Science," Chapter XI, pg. 317-318) "The isolation in which the Jews were forced to live, and the prohibitions long continued, against acquiring real estate, directed their speculations toward commerce and manufactures, in which they soon obtained incontestable superiority.... Nothing is more curious to study than the commercial condition of that nation which had no territory of its own, nor ports, nor armies, and which, constantly tacking about on an agitated sea, with contrary winds, at last arrived in port with rich cargoes and immense wealth. The Jews traded because it was rarely permitted them to employ themselves in any other way with security. While the multiplicity of toll-houses and the tyranny of the feudal lords rendered all trade impossible except that of the petty tradesmen of the market-towns and cities, the Jews, more bold, more mobile, were dreaming of vaster operations, and were working silently to bind together continents, to bring together kingdoms. They avoided the highways and the castles, carefully concealing their real opulence and their secret transactions under the appearances of poverty. They went great distances for rare products of the remote countries, and brought them within reach of well-to-do consumers. By wandering about and traveling from country to country they had acquired an exact acquaintance with the needs of all places; they knew where to buy and where to sell.