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The idealist tradition inaugurated by Plato led to the rise of universal, all-encompassing theories. That is, those which assert that there is a single unifying principle tying up together economics, politics, ethics, and social organization, and that this principle (whichever it may be) is suitable for any society at any time and place. Hence, Rousseaunianism, Socialism, Communism.
The "realist" tradition springing from Aristotle simply says that human problems can not be resolved by magical formulas or recipes. Social situations can not be severed from their immediate environment. Aristotle, then, classifies possible types of systems and defines their advantages and disadvantages for different types of societies. His approach, then, is that there can be no universal and general solutions or organizing principles. Aristotle is absolutely practical in his approach, as opposed to the theoretical systems imagined (as opposed to observed) by Plato. Hence: liberalism, Realpolitik, capitalism, democracy (or I should say "capitalisms" and "democracies", since there are very different varieties of these systems). Aristotle examines then distinct kinds of Constitutions, what they require to be effective, and what effects they might bring upon.
Read it, then, for a clear and well-ordered exposition of themes, subthemes, and advice. Here you will find the origin of half of Western political thought. And precisely the half that seems to be winning the race.
Aristotle's outline for government and state has been influential to political scientists for over 2,400 years. His discussion on the cons of complete unity, as well as his chapter on "the natural and unnatural methods of acquiring goods," certainly must have influenced Karl Marx, and his discussions on the "good of all" certainly led to Mills and Bentham's utilitarianism.
The Penguin Classics edition gives the reader an authoritative, inexpensive copy that is ideal for scholars as well as students. The footnotes are helpful, but not excessive. An excellent purchase all around.