From around 3000 BC, the Sumerian officials would mark their lists of goods on clay tablets. If they wanted to record five fish, they would mark five pictures of a fish. Their first great intellectual leap came from separating the number from the object they were counting. In other words, they would represent five fish with a numeral for the number five alongside a symbol for the fish. If they wanted to describe five of something else, they realized they could keep the same numeral and trade the object symbol for a cow, or a jar of oil, or whatever else they were interested in. The Sumerians had developed the idea of an emancipated number, existing in its own right and independent of whatever it is being used to count. It is easy to take the emancipated number for granted as it is so deeply set into modern thought, but to the earliest civilizations it was intellectually new and extremely powerful.