In economics, capital consists of assets that can enhance one's power to perform economically useful work. For example, a stone or an arrow is capital for a hunter-gatherer who can use it as a hunting instrument; similarly, roads are capital for inhabitants of a city. Capital is distinct from land and other non-renewable resources in that it can be increased by human labor, and does not include certain durable goods like homes and personal automobiles that are not used in the production of saleable goods and services. Adam Smith defined capital as "that part of man's stock which he expects to afford him revenue". In economic models, capital is an input in the production function.The total physical capital at any given moment in time is referred to as the capital stock (not to be confused with the capital stock of a business entity). Capital goods, real capital, or capital assets are already-produced, durable goods or any non-financial asset that is used in production of goods or services.