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The common good or common weal is a term that can refer to several different concepts. In the popular meaning, the common good describes a specific "good" that is shared and beneficial for all (or most) members of a given community. This is also how the common good is broadly defined in philosophy, ethics, and political science.
However there is no strict definition of the common good for each situation. The good that is common between person A and person B may not be the same as between person A and person C. Thus the common good can often change, although there are some things — such as the basic requirements for staying alive: food, water, and shelter — that are always good for all people.
Other definitions 
The common good has sometimes been seen as a utilitarian ideal, thus representing "the greatest possible good for the greatest possible number of individuals". In the best case scenario, the "greatest possible number of individuals" would mean all sentient beings. This definition of the common good presents it as a quality which is convertible, or reducible, to the sum total of all the private interests of the individual members of a society and interchangeable with them. But this is a narrow, and many would argue impoverished, view of what the common good might encompass.
Another definition of the common good, as the quintessential goal of the state, requires an admission of the individual's basic right in society, which is, namely, the right of everyone to the opportunity to freely shape his life by responsible action, in pursuit of virtue and in accordance with the moral law. The common good, then, is the sum total of the conditions of social life which enable people the more easily and straightforwardly to do so. The object of State sovereignty is the free choice of means for creating these conditions. Others, in particular John Rawls, make the distinction between the Good, that is actively creating a better world however that may be defined, and the Just, which creates a fair, liberal social infrastructure that allows the pursuit of virtue, but does not prescribe what the common good actually is.
Some assert that promoting the common good is the goal of democracy (in the sphere of politics) and socialism (in the sphere of economics). One of such philosophers of modern days is Michael Sandel of Harvard University. Ps: common good means to do what is best for the community as a whole.
One of the earliest references in Christian literature to the concept of the common good is found in the Epistle of Barnabas: "Do not live entirely isolated, having retreated into yourselves, as if you were already [fully] justified, but gather instead to seek together the common good."
The concept is strongly present in Augustine of Hippo's magnum opus City of God. Book XIX of this, the main locus of Augustine's normative political thought, is focused on the question, 'Is the good life social?' In other words, 'Is human wellbeing found in the good of the whole society, the common good?' Chapters 5-17 of Book XIX address this question. Augustine's emphatic answer is yes (see start of chap. 5).
Against that background, the common good became a central concept in the modern tradition of Catholic social teaching, beginning with the foundational document, Rerum Novarum, a papal encyclical by Pope Leo XIII, issued in 1891. This addressed the crisis of the conditions of industrial workers in Europe and argued for a position different from both laissez-faire capitalism and socialism. In this letter, Pope Leo guarantees the right to private property while insisting on the role of the state to require a living wage.
Contemporary Catholic social teaching on the common good is summarised in the 2004 Compendium of the Social Doctrine of the Church, chapter 4, part II. Quoting the Second Vatican Council document, Gaudium et Spes (1965), this says, "According to its primary and broadly accepted sense, the common good indicates 'the sum total of social conditions which allow people, either as groups or as individuals, to reach their fulfilment more fully and more easily'" (#164, quoting Gaudium et Spes, #26; italics original).
The Compendium later gives statements that communicate what can be seen as a partly different sense of the concept - as not only "social conditions" that enable persons to reach fulfilment, but as the end of goal of human life. "[T]he common good [is] the good of all people and of the whole person… The human person cannot find fulfilment in himself, that is, apart from the fact that he exists “with” others and “for” others" (#165; italics original). "The goal of life in society is in fact the historically attainable common good" (#168).
The Roman Catholic International Theological Commission drew attention to these two partly different understandings of the common good in its 2009 publication, In Search of a Universal Ethic: A New Look at the Natural Law. It referred to them as "two levels" of the common good.
Another relevant document is Veritatis Splendor, a papal encyclical by Pope John Paul II, issued in 1993 to combat the relaxation of moral norms and the political corruption (see Paragraph 98) that affects millions of persons. In this letter, Pope John Paul describes the characteristics and virtues that political leadership should require, which are truthfullness, honesty, fairness, temperance and solidarity (as described in paragraph 98 to 100), given that truth extends from honesty, good faith, and sincerity in general, to agreement with fact or reality in particular.
As an ethical and moral imperative, the common good can be seen as central to the tenets of many religious faiths. What it requires in practice can be succinctly described as doing unto others as we would wish done unto ourselves (known as The Golden Rule; cf. Matthew 7:12).
Contemporary American usage 
As regards contemporary American politics, the language of the common good (sometimes referred to as "public wealth") is increasingly being adopted by political actors of the progressive left to describe their values. Jonathan Dolhenty argues that one should distinguish between the common good, which may "be shared wholly by each individual in the family without its becoming a private good for any individual family member", and the collective good, which, "though possessed by all as a group, is not really participated in by the members of a group. It is actually divided up into several private goods when apportioned to the different individual members." First described by Michael Tomasky in The American Prospect magazine and John Halpin at the Center for American Progress, the political understanding of the common good has grown. The Take Back America Conference, the liberal magazine The Nation, and the Rockridge Institute have identified the common good as a salient political message for progressive candidates. More recently, the common good rhetoric is being used by political actors in an explicitly religious context, such as Kansans for Faithful Citizenship. In addition, non-partisan advocacy groups like Common Good  are also championing reform efforts to support the common good
See also 
- Sandel, Michael. "Justice (online course)".
- Epistle of Barnabas, 4, 10.
- Pontifical Council of Justice and Peace, Compendium of the Social Doctrine of the Church (2004), Chapter 4, part II
- International Theological Commission, In Search of a Universal Ethic: A New Look at the Natural Law (2009), #85
- Individualism and the Common Good
- Article | The American Prospect
- Reclaiming the Common Good
- A Politics of the Common Good
- Thinking Points — Rockridge Institute
- [dead link]