Human capital is a phrase coined by economists to describe education and training. Generally speaking, the higher the level of education, the more valuable the human capital. Indisputably, human capital is valuable, as it clearly and consistently increases human productivity. So human capital is, like physical capital, something that generates a stream of benefits, in the form of higher earnings that would not be possible without it. While human capital is most easily conceived as formal schooling or on-the-job training, there are clearly many other forms of human capital. Human capital may be the acquired knowledge of some facet of resource extraction, or some operational expertise connected to a specific industrial process. Like physical capital, human capital can be costly to acquire, not only because of direct costs, but because of the opportunity costs of time and of foregone income.