Reference has already been made to the wave of Arabic civilization which surged into Europe early in the Dark Ages. It was a civilization in every sense of the word; for in the ninth century learning had developed under the Caliphs of Baghdad to a degree unparalleled elsewhere in the world, and _rapprochements_ between the two races and civilizations, which had already begun in the world of philosophy, were soon strengthened and increased by those great medieval experiments, the Crusades.[36] Now Arabic scholars were, if anything, more enthusiastic Aristotelians than the scholars of Europe. The curious word _arabesque_, and the fact that words like _algebra_, _cipher_, _zero_, and some others to be mentioned in the next chapter are among the few Arabic words which reached our language before the fourteenth century, are both symptomatic of a certain peculiarity of the Arabic mind which we may perhaps call the tendency to abstraction. The Arab seems to have possessed something of that combination of materialism on the one hand and excessive intellectual abstraction on the other which we have already noticed in the later stages of Roman mythology. Just as he made Mohammedanism out of the Jewish sacred traditions, so he made Nominalism out of Greek philosophy. The influence upon Christian thought of great Arabic philosophers like Averroes and Avicenna is one of the most astonishing chapters in its history. But it is not difficult to see how it occurred. The learning of the Middle Ages was founded entirely on translations, and this was an activity in which, as far as Aristotle’s works were concerned, the Arabs had got in first. According to Renan, some of the current versions of Aristotle were “Latin translations from a Hebrew translation of a Commentary of Averroes made on an Arabic translation of a Syriac translation of a Greek text”.