Treatise on Law

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Treatise on Law is St. Thomas Aquinas' major work of legal philosophy. It forms questions 90-108 of the Prima Secundæ ("First [Part] of the Second [Part]") of the Summa Theologiæ,[1] Aquinas' masterwork of Scholastic philosophical theology. Along with Aristotelianism, it forms the basis for the legal theory of Catholic canon law.[2]

Aquinas' notion of law

Aquinas defines a law as "an ordinance of reason for the common good, made by him who has care of the community, and promulgated."[3]

Law is an ordinance of reason because it must be reasonable[4] or based in reason and not merely in the will of the legislator.[5][6] It is for the common good because the end or telos of law is the good of the community it binds, and not merely the good of the lawmaker or a special interest group.[7] It is made by the proper authority who has "care of the community", and not arbitrarily imposed by outsiders. It is promulgated so that the law can be known.

He says:

Thus from the four preceding articles [of Question 90], the definition of law may be gathered; and it is nothing else than an ordinance of reason for the common good, made by him who has care of the community, and promulgated.[8]

Strictly speaking, this is a definition of human law.[9] The term "law" as used by Aquinas is equivocal, meaning that the primary meaning of law is "human law" but other, analogous concepts are expressed with the same term.[10]

Kinds of law

Natural law

Natural law or the law of nature refers to normative properties that are inherent by virtue of human nature, and universally cognizable through human reason. Historically, natural law refers to the use of reason to analyze both social and personal human nature to deduce binding rules of moral behavior. The law of nature, being determined by nature, is universal.

Human law

For Aquinas, human law is only valid if it conforms to natural law. If a law is unjust, then it is not actually a law, but is a "perversion of law".[11][12]


Summa theologica, Pars secunda, prima pars. (copy by Peter Schöffer, 1471)

The Treatise on Law (as part of the Summa Theologica) is divided into Articles (or broad topics) and Questions (or specific topics). The Questions each argue for a single thesis and defend it against objections. The division is as follows:[13]


Q. 90: Of the Essence of Law (the rationality, end, cause, and promulgation of law)
Q. 91: Of the Various Kinds of Law (eternal, natural, human, divine, sin laws)
Q. 92: Of the Effects of Law


Q. 93: Of the Eternal Law
Q. 94: Of the Natural Law
Q. 95: Of Human Law
Q. 96: Of the Power of Human Law
Q. 97: Of Change in Laws
Q. 98: Of the Old Law
Q. 99: Of the Precepts of the Old Law
Q. 100: Of the Moral Precepts of the Old Law
Q. 101: Of the Ceremonial Precepts in Themselves
Q. 102: Of the Causes of the Ceremonial Precepts
Q. 103: Of the Duration of the Ceremonial Precepts
Q. 104: Of the Judicial Precepts
Q. 105: Of the Reason for the Judicial Precepts
Q. 106: Of the Law of the Gospel, Called the New Law, Considered in Itself
Q. 107: Of the New Law as Compared with the Old
Q. 108: Of Those Things That Are Contained in the New Law


William S. Brewbacker III has called it "perhaps the most famous of metaphysical legal texts”,[14] while Robert M. Hutchins declared it “that greatest of all books on the philosophy of law”.[15]

See also


    James Fieser
    Journal of Philosophical Research, 1992, Vol. 17, pp. 147-164.
    accessed Dec-17-2013
  2. Dr. Edward N. Peters,, accessed Dec-17-2013
  3. Gateway Edition, pg. ix
  4. Law of Christ I, pg. 236
  5. J. Budziszewski, Commentary on Treatise on Law, accessed Dec-19-2013
  6. Gateway Edition, pg. 2 (Summa, Ia-IIæ, Q.90, A.1, Obj.3)
  7. Law of Christ I, pg. 236
  8., accessed Dec-19-2013
  9. Gateway Edition, pg. viii
  10. Gateway Edition, pg. x
  11. Summa I-II, q95, a2, 'dicendum quod
  12. Internet Encyclopedia of Philosophy, "Natural Law", accessed Dec-20-2013
  13., accessed Dec-17-2013
  14. Brewbaker, Metaphysics of Law, 575.
  15. Hutchins, World State, 38.


  • Aquinas, St. Thomas. Treatise on Law (Summa Theologica, Questions 90-97): With a New Introduction by Ralph McInerny, University of Notre Dame. (Washington, D.C.: Gateway Editions, Regnery Publishing, Inc. ©1956; 2001 printing).
  • Brewbaker, William S., III. Thomas Aquinas and the Metaphysics of Law (Alabama Law Review, Vol. 58 [2007], pg. 575); U of Alabama Public Law Research Paper No. 898941; Social Sciences Research Network: Accessed 28 March 2016.
  • Häring, Bernard, C.SS.R. The Law of Christ, Vol. I. Translated by Edwin G. Kaiser, C.PP.S. (Westminster, Maryland: The Newman Press ©1961, Second Printing November 1961).
  • Hutchins, Robert M. Aquinas Lecture 1949: St. Thomas and the World State (Milwaukee: Marquette University Press, 1949).