In this paper, I argue that Aristotle was the first and only philosopher to have developed what may be called a general theory of citizenship. This theory is useful in order to assess the coherence and consistency of policies regulating the access to the polity that include most notably citizenship policy, but also migration policies regulating prior segments of the route to naturalisation. The theory is based on the thesis of correlation, according to which access to status civitatis is a variable of the content of the status. There is, in other words, a relationship between who is a citizen (i.e. the extension of citizenship) and what a citizen is (i.e. the intension of citizenship). I think this relationship is best described as a functional correlation, even if these are not the Aristotelian terms. Function here appears in its mathematical meaning. The correlation is functional, in the sense that the criteria regulating acquisition and loss of the status must fit the entitlements or legal positions that citizenship gives access to. If the criteria for acquisition and loss are not aligned with the legal positions associated with citizenship, the status becomes an arbitrary tool of social closure. This theory, I argue, focuses on the functional dependence between legal positions (rights and/or obligations that constitute the content of status civitatis) and the criteria for acquisition and loss of the status (which gives the form to the status). To answer the question «who should be a citizen?» it is necessary to ask whether it is reasonable that a certain personal feature is deemed necessary and/or sufficient to confer the status which gives access to a given set of entitlements. If the answer is positive, the feature constitutes a reason for conferring citizenship; otherwise, it does not. For Aristotle, the person who can exercise the rights and fulfil the duties of citizenship is entitled to the status. Which characteristics are relevant (and which should be reasonably required) will depend on the rights and duties related to a specific citizenship. The latter are not the same everywhere because they depend on the constitutional identity of the state. Where the state is democratic, citizenship rights include participating in popular juries, but not so elsewhere. The criteria for acquisition and loss of citizenship also change, depending on the constitutional identity of the state
