The natural law tradition as explicated by Saint Thomas Aquinas is foundational for Catholic medical ethics. Here is a very brief description of the Natural Law theory of Thomas Aquinas as it affects that field of moral theology. Included in the description are (a) the historical antecedents of Natural Law, (b) the remote and proximate foundation of Natural Law, (c) the specification of the first principle and precept of Natural Law, (d) the formulation of the specific principles of Natural Law, and (e) the limitations as the movement to made from general principles to contingent action, and (f) the response of the Natural Law position to a particular issue in medical ethics.

According to St. Thomas, the natural law is “nothing else than the rational creature’s participation in the eternal law.”[1] His natural law theory posits that the eternal law as it is in the mind of God is knowable in a limited degree by our finite human intellects; or, as Dr. Hogan says “(1) there is a natural order or rule in the universe and (2) this natural order or rule is accessible to human reason as (a) known by reason and (b) as directed by reason.”[2]

(a) The historical antecedents to the natural law theory of St. Thomas are the deposit of Faith and the philosophical tradition of Roman Stoicism. In regard to the deposit of faith, the two-fold rule of Scripture and Tradition witnesses to the presence of a law of God in which men participate by their rational nature. St. Paul thus contrasts the Gentiles’ knowledge of God through nature with the Jews’ knowledge through revelation: “For when the Gentiles, who have not the law, do by nature those things that are of the law; these having not the law are a law to themselves.”[3] Sources of apostolic tradition, such as the writings of St. Augustine, St. Basil, and St. John Damascene also contribute to St. Thomas’ reasoning, and are explicitly referenced in the Summa. The thinking of Greek and Roman philosophers, especially the Roman Stoics (among whom, Julius Caesar is referenced[4]) also contributes. The works of Aristotle, the medieval Aristotelian philosopher Boethius (also a source of Tradition), and the then-extant code of canon law (the Decretals) are also referenced.

(b) The remote source of Natural Law is the Eternal Law, which is God’s Providence as governing creation. Its proximate source is human reason “as human reason comes to understand human nature.”[5] God ordered all of creation to the achievement of certain ends and put in each creature the means by which it could attain its end. Man, a being endowed with reason, has in himself the means by which he can tend to the end proper to a rational creature. Included among these means is the natural law.

(c) In the words of St. Thomas, “the first principle of practical reason is one founded on the notion of good, viz. that ‘good is that which all things seek after.’ Hence this is the first precept of law, that ‘good is to be done and pursued, and evil is to be avoided.’ All other precepts of the natural law are based upon this: so that whatever the practical reason naturally apprehends as man’s good (or evil) belongs to the precepts of the natural law as something to be done or avoided.”[6]

(d) Perceiving the goodness of the natural inclinations of his rational nature, the practical intellect treats them as the principles from which to formulate the more specific precepts of the natural law, the first of which is based on man’s tendency to self preservation. This first principle is “preserve life.”

(e) The movement from general principles and precepts of the Natural Law to specific human acts is “fraught with difficulty”[7] because of the complexity of human action. It could be that more than one of the secondary principles of Natural Law comes into play in a given instance, and it could be that the contingencies and complex processes of secondary causality make the right course of action anything but manifest. Such complications can and do enter into the sphere of Medical Ethics.

(f) It could be that someone in a hospital has a condition which would be worsened by the intake of fluids, such as kidney failure or congestive heart failure. Prudent medical practice, applying the principle “preserve life,” could dictate that such a one be very restricted in his fluid intake (at least until diuretics or dialysis can be administered). These conditions also cause great thirst. Now, the natural law puts in the patient an instinct of self-preservation, telling him it is better to drink than not to drink. However, the known medical fact that hydration at this time could be detrimental, must restrain the impulse of patient to drink. This is so because of the principle which says that life should be maintained and death avoided is on the side of restricting fluid intake. In this example we not only see an illustration of the secondary principle of Natural Law (preserve life), but we see an example of how (at least in the patient’s mind), the precise application of the law can be “fraught with danger” as, in his mind, drinking large quantities of water is reasonable.

[1]. ST I-II, Q. 91, A. 2.

[2]. Dr. Hogan, class notes for second lecture,

[3]. Rom. 2:14.

[4]. ST I-II, Q. 91, A.

[5]. Dr. Hogan, op. cit.

[6]. ST I-II, q. 94, a 2.

[7]. Dr. Hogan, op. cit.