Production requires inputs of land, labor, capital, and entrepreneurship to create outputs of goods and services. Land includes natural resources while labor consists of human physical and mental work. Capital refers to manufactured goods used to produce other goods. Inputs are either fixed, remaining constant regardless of production levels, or variable, changing with the quantity produced. The law of diminishing returns states that as variable inputs are increased with fixed inputs held constant, total product increases but at a decreasing rate, with smaller increases in output for each additional unit of input. Marginal product is the additional output from adding a unit of a variable input while holding other inputs fixed.