The power of an individual or institution is the ability to achieve something, whether by right or by control or influence. Power is the ability to mobilize economic, social, or political forces in order to achieve a result. It can be measured by the probability of that result being achieved in the face of various kinds of obstacle or opposition. It is not essential to this definition that the result be consciously intended by the powerful agent: power may be exercised unknowingly, although of course it is frequently deliberate. However, in the views of some theorists, notably Foucault, all social relations are systems of power: fundamental power is not exercised by individuals, but is a dispersed, impersonal aspect of society, and in particular is manifested in the modes of surveillance, regulation, or discipline that adapt human beings to the surrounding social structure. The power of society is not limited to its ability to prevent people doing things, but it includes control of the self-definition and preferred way of living of its members. A principal concern of political theory is to determine when the exercise of power is legitimate; this is often posed as the problem of distinguishing authority from power. See also exploitation, oppression.