The common good 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.
The common good is often regarded 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.
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, makes 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.
The common good is a concept central to the Catholic social teaching tradition beginning with the foundational document, Rerum Novarum, a papal encyclical by Pope Leo XIII, issued in 1891 to combat the excesses of both laissez-faire capitalism on the one hand and communism on the other. In this letter, Pope Leo guarantees the right to private property while insisting on the role of the state to require a living wage. The means of production were considered by the pope to be both private property requiring state protection and a dimension of the common good requiring state regulation.
- 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. This greatly changed romes political system.
Application in American Politics
Increasingly, progressive Americans are adopting the language of the common good (sometimes referred to as "public weal") to describe progressive values. As an ethical and moral imperative, the common good is central to the tenets of many religious faiths and can be succinctly described as doing unto others, to use a Biblical phrase, as we would wish done unto ourselves. Aristotle was the first to articulate an ethical understanding of common good, followed by Augustine of Hippo and Thomas Aquinas who developed the concept into standard moral theology.
Concerning contemporary American politics, the common good language is increasingly identifiable with political actors of the progressive left. 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.