During the late nineteenth and early twentieth centuries fundamental changes in economic thought revolutionized the theory of corporate finance, leading to changes in its legal regulation. The changes were massive, and this branch of financial analysis and law became virtually unrecognizable to those who had practiced it earlier. The source of this revision was the marginalist, or neoclassical, revolution in economic thought. The classical theory had seen corporate finance as an historical, relatively self-executing inquiry based on the classical theory of value and administered by common law courts. By contrast, neoclassical value theory was forward looking and as a result a much more realistic way of assessing a corporation's value; but it was also subject to a great deal more prediction and interpretation, and thus to more abuse. That possibility led the states first and later the federal government to respond with regulatory legislation.While marginalism effected a sweeping change in regulatory attitudes toward the corporation, the changes in the basic theory of corporate behavior, including finance, were at least as striking. The marginalist revolution turned the corporation into a rational actor intent on maximizing value. Neoclassical corporate finance theory unambiguously choose marginalist price theory rather than welfare economics as the source of its working assumptions, thus guaranteeing the irrelevance of not only the individual shareholder but also of managers. Or to say it differently, the neoclassical concept of the corporation did not merely separate ownership from control; it separated corporate decision making from all human preference whatsoever, unless those preferences were simply asserted to be maximization of value. Within the neoclassical model the separate human identities of shareholders or even managers came to matter only under the rubric of agency costs, which were regarded as nothing more than an imperfection in the neoclassical corporate ideal.