Starting from the fact that in the classification of the constitutions, illustrated by Aristotle in the Politics, the right constitutions are those in which the laws are observed, the article intends to examine the role of the law in the definition and functioning of a constitution. Since each constitution is divided into three parts, that is, the deliberative function, the executive function performed by the magistrates and the judging function, the article examines whether and how the laws are applied in the exercise of each of these functions. As regards the legislation, Aristotle does not exclude the possibility of entrusting to the legislative bodies of the city the right to change the laws if circumstances so require. He believes, however, that even the smallest change of the laws is capable of causing an unwanted change of the constitution.
Regarding the public offices, Aristotle emphasizes the power to command through decrees that must comply with the laws. Regarding the judicial function, Aristotle, after having classified the main types of tribunals, affirms that even the judges must observe the laws as much as possible. The article also examines the institutional remedies that Aristotle suggests to mitigate the imbalance of laws in favour of the socially and economically dominant component in the corresponding political regime.