The invention of reality
The Romans were excellent engineers, as were many other ancient civilizations, but the Greeks were the first scientists. I've commented on this in other posts, but it cannot be over-emphasized. They invented the modern concept of Nature, an impersonal objective reality operating on its own rules, which are nevertheless understandable by the use of human reason. It's what made Western European civilization fundamentally different from the Egyptians, Mesopotamians, Arabs, Persians, Hindus, Chinese, all the others.