Natural functions do not automatically create duties
Critics question the move from biological or characteristic inclinations to binding moral norms.
Medieval philosophy · c. 1225–1274
Aquinas
Aquinas joins Aristotelian explanation with Christian theology, arguing that reason can identify natural ends, moral principles and routes towards a first cause.
Thomas Aquinas was an Italian Dominican friar, theologian and philosopher. Drawing extensively on Aristotle, he constructed a systematic account of being, causation, knowledge, natural law, virtue and God that became foundational for the Thomistic tradition.
EthicsKnowledgeAgencyJustice
The question he enters
The question Aquinas enters
Can facts about what human beings are and what they naturally seek tell us how they ought to act?
The central move
Read moral order through nature and reason
Aquinas holds that rational creatures participate in eternal law by recognising basic human goods and directing action towards them. Natural law begins from the practical principle that good is to be pursued and evil avoided, then develops through precepts concerning life, community, knowledge and rational flourishing.
Key ideas
Terms that carry the argument.
- Natural law
- Rational participation in moral order through practical principles grounded in characteristic human goods and inclinations.
- Primary precepts
- General requirements directed towards goods such as preserving life, living in society, educating offspring and seeking truth.
- Doctrine of double effect
- A later Thomistic principle distinguishing an intended good effect from a foreseen harmful side effect under strict conditions.
- Five Ways
- Arguments from change, causation, contingency, gradation and order towards a first explanatory source identified with God.
Where the view strains
Objections and limits.
First causes need further identification
Even if a cosmological argument reaches a necessary or first cause, more argument is needed to establish the attributes of the God of classical theism.
Sources and routes
Read the work. Follow the argument.
Primary texts
Where to meet the position.
- New AdventSumma TheologiaeWebA systematic treatment of God, human action, law, virtue and Christian doctrine.English Dominican Province translation, second revised edition, 1920; online edition presented by New Advent. · Free article-by-article HTML text. Checked Aug 2026.
- Christian Classics Ethereal LibrarySumma Contra GentilesPDFArguments concerning God, creation, providence and rationally accessible theology.Joseph Rickaby translation and annotations, published online under the title Of God and His Creatures. · Free readable HTML with PDF, EPUB and other download options supplied by CCEL. Checked Aug 2026.
- Iowa Research OnlineOn Being and EssenceWebA concise metaphysical analysis of essence, existence and created being.University of Iowa repository record for Robert T. Miller's English translation in Selected Writings of St. Thomas Aquinas (Fordham University Press, 1997). · Institutional bibliographic record with an external open-access link; access to the linked text may depend on the destination site. Checked Aug 2026.