In ecological economics value theory is separated into two types: donor-type value and receiver-type value. Ecological economists tend to believe that 'real wealth' needs a donor-determined value as a measure of what things were needed to make an item or generate a service (H. T. Odum, "Environmental Accounting: Emergy and environmental decision-making", 1996). An example of receiver-type value is 'market value', or 'willingness to pay', the principal method of accounting used in neo-classical economics. In contrast, both Marx's labour theory of value and the emergy concept are conceived as donor-type value. Emergy theorists believe that this conception of value has relevance to all of philosophy, economics, sociology and psychology as well as Environmental Science.