Ancient Egypt was the civilization that formed around the Nile River. The Egyptians developed a distinctive system of writing, along with a system of decimal numbers. Egyptian counting on the fingers is mentioned in the Book of the Dead, which was written around 1600 BCE but was based on texts perhaps a thousand years older. The Greek historian Herodotus described the Egyptians as using pebbles to calculate, a form of abacus whose results were written in one of three scripts: small pictures and conventions known as hieroglyphs; a cursive form of hieroglyphs called hieratic; and a more abstract form named demotic. In numbers, the transition from hieroglyphs to hieratic to demotic forms shows how conventional forms for numbers (that is, signs for numbers whose meaning cannot be ascertained by counting the number of elements in the way a collection of vertical strokes can be counted) can emerge from handwriting and perceptual effects. One such perceptual effect influences the form of small numbers (one through three, consisting of one, two, or three vertical strokes) to be conserved, while higher numbers (four and above) change away from countable elements.