energy. In his own words, "culture evolves as the amount of energy harnessed per capita per year is increased, or as the efficiency of the instrumental means of putting the energy to work is increased". Nikolai Kardashev extrapolated his theory, creating the Kardashev scale, which categorizes the energy use of advanced civilizations. Lenski's approach focuses on information. The more information and knowledge (especially allowing the shaping of natural environment) a given society has, the more advanced it is. He identifies four stages of human development, based on advances in the history of communication. In the first stage, information is passed by genes. In the second, when humans gain sentience, they can learn and pass information through experience. In the third, the humans start using signs and develop logic. In the fourth, they can create symbols, develop language and writing. Advancements in communications technology translate into advancements in the economic system and political system, distribution of wealth, social inequality and other spheres of social life. He also differentiates societies based on their level of technology, communication, and economy: hunter-gatherer, simple agricultural, advanced agricultural, industrial, special (such as fishing societies). In economics, productivity is a measure of technological progress. Productivity increases when fewer inputs (classically labor and capital but some measures include energy and materials) are used in the production of a unit of output. Another indicator of technological progress is the development of new products and services, which is necessary to offset unemployment that would otherwise result as labor inputs are reduced. In developed countries productivity growth has been slowing since the late 1970s; however, productivity growth was higher in some economic sectors, such as manufacturing. For example, employment in manufacturing in the United States declined from over 30% in the 1940s to just over 10% 70 years later. Similar changes occurred in other developed countries. This stage is referred to as post-industrial. In the late 1970s sociologists and anthropologists like Alvin Toffler (author of Future Shock), Daniel Bell and John Naisbitt have approached the theories of post-industrial societies, arguing that the current era of industrial society is coming to an end, and services and information are becoming more important than industry and goods. Some extreme visions of the post-industrial society, especially in fiction, are strikingly similar to the visions of near and post-Singularity societies. By period and geography The following is a summary of the history of technology by time