IN AN OFT-QUOTED DEFINITION, St. Thomas Aquinas defines law as "nothing other than a certain dictate of reason (rationis ordinatio) for the common good, made by him who has the care of the community and promulgated." ST. IaIIae, Q.90, art.4. Perhaps the most significant emphasis of this definition is that law is based upon reason, and not upon custom, will, politics, or power. Therefore, St. Thomas is clearly outside the voluntaristic camp, those who, like the medieval Ockham, sought the essence of law in God's will as distinguished from His reason. Similarly, he departs from those who suggest a command theory of law, the most notable of these being perhaps John Austin (1790–1859) who, seeking to disassociate the law from precepts of morality, in his The Province of Jurisprudence Determined (1832) defined law as the command of the sovereign backed by sanctions. St. Thomas Aquinas's emphasis on the practical reason as a source of substantive values also distinguishes him from philosophers such as David Hume, who rejected the role of practical reason in legislation, as he found it only instrumental ("Reason is, and ought only to be the slave of the passions," Treatise of Human Nature, 188.8.131.52). Similarly, the German jurisprudential scholar Rudolf von Jhering, who, in his Der Zweck im Recht, saw law as merely a "means to an end" without substantive component other than the convenience of the society works outside St. Thomas's frame of reference. Although St. Thomas recognizes the role of customary law, in it focal meaning, law is more than that which merely what steps out of the practice (Übung), conventions (Sitte), and customs (Gewonhneit) of a society, something that rises from an inner, silently-working forces, the innere, stillwirkende Kräfte, of a people's consciousness. St. Thomas would therefore not be in agreement with the likes of Frederich Carl von Savigny. The concept of law envisioned by St. Thomas radically departs from the Critical Legal Studies school which claims that law is indeterminate, and ultimately that "law is politics" and nothing else. Similarly, Michel Foucalt's view that all law is nothing but power is rejected by St. Thomas Aquinas in his definition of law.
It is not that these other theories do not have some truth to them, and perhaps in some fashion they even adequately explain law, or its abuse, or a certain characteristic of it, in certain times and places. They may be valuable in ascertaining law in a loose sense, or in a sense which is not its focal sense, i.e., its sensu lato. For St. Thomas, however, the issue was what the definition of law was in sensu stricto, in its principal focal meaning. For St. Thomas, the kernel, heart, and soul of the law is reason, above all other things. Thus, a "law" may be a command of the sovereign, may be the result of the political process, may be issued by the powerful against the weak, may be perfectly customary, and yet--if it does not conform to reason--it is not law in the strict sense. Rather, though it may have indicia of being a "law," it ought, in fact, be considered tantamount to violence. ST IaIIae, Q. 93, art. 3, ad. 2.