One of the major puzzles in the history of the Graeco-Roman world lies in the great discrepancy between its considerable achievements in art, in literature, in philosophy, in mathematics, and in medicine, and its very marked backwardness in most branches of technology. This is less surprising where the Romans are concerned, since, for all their admirable qualities in other directions, they were not a conspicuously original or inventive people. Even in those spheres in which they most excelled, the arts of government and warfare, they made few contributions of their own, and their strength lay rather in their skill at adapting to their own purposes the bright ideas of other men. It is significant that those two simple but important aids to improved horsemanship and cavalry tactics, the saddle and the stirrup, were not invented by the Romans, nor indeed by the Greeks, but by the nomadic tribes that pressed in on the Roman empire from the third century A.d. onwards. But with the Greeks the case is different. They were a highly intelligent people, gifted with a degree of inquisitiveness which made them unwilling to accept without question the outward appearance of the world in which they lived. Consequently in mathematics and certain branches of pure science they were able to make quite astonishing progress. But these intellectual advances were not accompanied by any marked degree of technological improvement. While Eratosthenes in Alexandria was calculating the circumference of the earth, and obtaining a figure that was less than i per cent short of the real one, the world around him was still more or less at the same technical level as it had been since the end of the Bronze Age. This contrast between theoretical brilliance and practical incompetence is great and dramatic, and it is the purpose of this paper to suggest some of the reasons why it existed.