One of the central questions of political philosophy is what the purpose of government is. Many great political philosophers, from Plato to John Rawls, have concerned themselves with this question.

One common formulation is that the purpose of the state is to protect rights and to preserve justice. A countervailing formulation is that government exists to protect privileges of a few, preserving a state of injustice for the majority. But this raises more questions than it answers. Which and whose rights? What sort of justice? There are, after all, many different conceptions of what rights are, and what justice consists of.

It is on those questions that one can find the differences between conservatism, socialism, liberalism, communism, libertarianism, fascism and other political philosophies. There is also anarchism, of course, which rejects the state altogether, but everyone else agrees that the existence of some kind of government is morally justified. What they disagree about is what government should do.

There are several ways to conceive of the differences between these different political views. For example, one might ask in what areas should the government have jurisdiction, to what extent it may intervene in those areas, or even what constitutes intervention in the first place (a lot of institutions can be said to exist only because the government provides the framework for their existence; for instance, Marxists argue that the institution of private property only exists due to government).

The criteria mentioned above often serve to distinguish political views on the political spectrum, or the left/right axis.

The constitutions of various countries codify practical views as to the purposes of their governments, but they tend to do so in rather vague terms, which particular laws, courts, and actions of politicians subsequently flesh out. For example, the Preamble of the United States Constitution states that the purpose of the Constitution--which defines the American state--is "to form a more perfect union, establish justice, ensure domestic tranquility, provide for the common defense, promote the general welfare, and secure the blessings of liberty to ourselves and our posterity." The phrase "promote the general welfare" has, since the 1930s, been used to defend the proposition that the United States should implement a social "safety net", or welfare system; others, however, have disagreed that the phrase can be properly interpreted that way, pointing to the use of "promote" as opposed to "provide". Since the act of "promoting" something is not clearly defined, however, the whole matter remains controversial - and the same applies to the other declared aims of the US government, such as "forming a more perfect union" or "securing the blessings of liberty". In general, various countries have translated vague talk about the purposes of their governments into particular state laws, bureaucracies, enforcement actions, etc.

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