Editing Price elasticity of demand (section)

==History==
[[File:Marshall PED.png|thumb|right|The illustration that accompanied Marshall's original definition of elasticity, the ratio of PT to Pt]]

Together with the concept of an economic "elasticity" coefficient, [[Alfred Marshall]] is credited with defining  "elasticity of demand"  in ''[[Principles of Economics (Marshall)|Principles of Economics]]'', published in 1890.<ref>Taylor, John (2006). p.&nbsp;93.</ref> Alfred Marshall invented price elasticity of demand only four years after he had invented the concept of elasticity. He used Cournot's basic creating of the demand curve to get the equation for price elasticity of demand. He described price elasticity of demand as thus: "And we may say generally:— the elasticity (or responsiveness) of demand in a market is great or small according as the amount demanded increases much or little for a given fall in price, and diminishes much or little for a given rise in price".<ref>Marshall, Alfred (1890). III.IV.2.</ref> He reasons this since "the only universal law as to a person's desire for a commodity is that it diminishes ... but this diminution may be slow or rapid. If it is slow... a small fall in price will cause a comparatively large increase in his purchases. But if it is rapid, a small fall in price will cause only a very small increase in his purchases. In the former case... the elasticity of his wants, we may say, is great. In the latter case... the elasticity of his demand is small."<ref>Marshall, Alfred (1890). III.IV.1.</ref> Mathematically, the Marshallian PED was based on a point-price definition, using differential calculus to calculate elasticities.<ref>Schumpeter, Joseph Alois; Schumpeter, Elizabeth Boody (1994). p.&nbsp;959.</ref>