Aquinas’s Commentary on Aristotle’s Politics 1.1.1-3
Philosophy, at least etymologically, is the love of wisdom, and so Aquinas begins his commentary on Aristotle’s Ethics by defining wisdom as the knowledge of order, or of how things relate to each other (§1). Because knowledge involves human reason, the principle of order Aquinas uses to organize the different philosophical sciences are the four possible ways he sees the subject matter of a given science as being able to relate to human reason. The first type of order is the objective, extra-mental order of things that human reason does not establish but merely observes in the nature of things. The sciences which study this order of things Aquinas classifies under the heading of “natural philosophy” (§1-2). Second, there is an intra-mental order that reason is able to establish among its own concepts or ideas, the study of which belongs to “rational philosophy,” or what we might call logic. Third, there is an order that practical reason, in its own act of deliberation, seeks to establish within the operations of the human will and action, and is the subject matter of “moral philosophy.” Fourth and finally is the order that human reason is able to establish in the external things of human making or production, the study of which fall under the heading of the “mechanical arts.”
Both the sciences of ethics and politics, of course, belong to moral philosophy, or the science of human actions in their ordering towards each other and towards their end. By human actions, moreover, Aquinas wishes to make clear that he is speaking of specifically those human activities that are a matter of human will and reason, for those activities, such as breathing or digestion, are shared with other creatures and are therefore not uniquely or properly human (§3). The subject of moral philosophy, therefore, is human action insofar as it is voluntary and acting for an end.
The Natural Law Libertarian 