CHAPTER ONE

INTRODUCTION

1.1 Background to the Study

From antiquity, human beings have been described as rational animals. Speech, inventiveness, art, politics, religion and morality are all commonly traced to the capacity for reason, and it is this capacity that is thought to set humans apart from the rest of the animal world. Aristotle gave the idea its classic form by treating reason as the highest power of the human soul and the basis of distinctively human flourishing (Aristotle, Nicomachean Ethics, Book I, ch. 7; Book X).

The link between reason and morality has remained one of the central questions of ethics. Is reason required for a being to be moral? If so, is it the only factor, or do emotion, habit and will also matter? Hume gave a well-known negative answer. For him reason alone cannot move the will, being instead the servant of the passions, and moral distinctions arise from feelings of approval and disapproval and not from rational inference (Hume, A Treatise of Human Nature, Book II, Part III, section 3; Book III). Against this stands a tradition that treats moral standards as something reason discovers and applies, and Thomas Aquinas is its most influential representative.

Aquinas is so central to this tradition that contemporary scholars use his theory as the reference case for what counts as a natural law view of ethics (Murphy, 2025). Murphy identifies two key features of the theory. Seen from God's side, the natural law is one aspect of divine providence: it is the way rational creatures participate in the eternal law, the rational plan by which creation is ordered. Seen from the human side, it supplies the basic principles of practical rationality, the standards by which human action is judged reasonable or unreasonable (Murphy, 2025, discussing Summa Theologiae I-II, q. 91, a. 2 and q. 94, a. 2). Its fundamental precept is that good is to be done and pursued and evil avoided, and the goods Aquinas names include life, procreation, knowledge, life in society and reasonable conduct (Murphy, 2025).

This outlook is captured in a principle often associated with Aquinas, that the good of the human being lies in living according to reason (bonum hominis est secundum rationem esse). It does not mean that human conduct is governed by reason alone. Aquinas recognises that will, habit, passion and emotion also shape behaviour. Strong emotion or vicious dispositions can obstruct the effect of the knowledge reason supplies, and because the particulars of a situation always outrun general rules, the moral and intellectual virtues are needed to act well (Murphy, 2025). Even so, his account is distinctly intellectualist, since it makes reason the measure of right action.

Recent scholarship shows that this reading is neither simple nor settled. Vaccarezza (2017) argues that Aquinas's treatment of reason developed over time: his earlier works stress the priority of the habit by which the first practical principles are known, whereas his mature works pair natural law with prudence and describe a cooperation between reason, natural inclinations and virtue. Brock (2020) shows that Aquinas's natural law teaching is saturated with metaphysics, because it concerns first principles of practical reason that function as laws and are ultimately related to the divine will. A continuing question in this literature is whether Aquinas's ethics should be read as heteronomous, since it depends on God and nature, or as a form of autonomy in which practical reason itself establishes moral norms (Vaccarezza, 2017).

This study works within that debate. It asks whether and how reason guides morality in Aquinas, and what that claim implies for the moral life. The work is organised in four chapters.

1.2 Statement of the Problem

There are actions that people individually and collectively condemn as wrong, such as murder, robbery, embezzlement of public funds, bribery and corruption. There are others, such as kindness, hospitality and honesty, that are widely regarded as good. There is also a third group, including abortion, contraception, euthanasia and nuclear warfare, on which agreement is lacking because intelligent people disagree about their rightness or wrongness.

These observations raise questions about Aquinas's philosophy in particular. Why does he count some actions as good and others as bad? What yardstick does he use to measure them, and what does it mean, on his assumptions, to call an action good or bad in itself? Is it our thinking that makes an action good or bad, or does the moral quality belong to the action?

Philosophers disagree sharply about the answer. Hume held that reason is and ought to be the servant of the passions, so that morality is determined by sentiment. Natural law thinkers, Aquinas pre-eminent among them, hold instead that right reason supplies the moral standard. In Aquinas's view an act is right when it is in no way defective as an action, and its defects are found by examining features such as its object, its end and its circumstances (Murphy, 2025, discussing Summa Theologiae I-II, q. 18). Some other thinkers in the tradition, such as Duns Scotus, accept that human nature fixes the content of the natural law but locate its binding force in divine command (Murphy, 2025).

A difficulty remains even for those who accept right reason as the standard: how does one know whether a particular practice conforms to it? Aquinas holds that some acts, such as the killing of the innocent and lying, are always wrong, yet he does not appear to supply a single master principle for determining which acts are defective (Murphy, 2025). On contested matters such as abortion, euthanasia and contraception, cogent arguments are offered on both sides. The problem this study addresses is therefore twofold: what role Aquinas assigns to reason in morality, and whether reason, so understood, can settle the hard cases.

1.3 Purpose of the Study

The main purpose of this study is to analyse critically the notions of morality and reason and to show, with particular reference to Thomas Aquinas, how morality is guided by reason. It also aims to help students in Nigerian universities and other tertiary institutions to understand more clearly the relationship between morality and reason, and to clarify the ideas, principles and benefits associated with reasoned moral thinking.

1.4 Significance of the Study

The study will present a clear account of how morality and reason are related in Aquinas, and it will help to distinguish the ancient and medieval conception of this relationship from more modern ones, such as Hume's. It is hoped that readers who understand their rationality and moral values better will be more able to contribute to a society in which immorality and irrational conduct are checked. The study will also expose some of the weaknesses in the way human beings approach morality and reason, and it will serve as a reference for later students of ethics and of Thomistic philosophy.

1.5 Scope of the Study

The study is confined to Aquinas's interpretation of morality in relation to reason, with attention to how he connects moral action with human nature and the human environment. Competing views of morality and reason, including the sentiment-based view associated with Hume, receive attention to the extent needed to assess Aquinas's position critically.

1.6 Methodology of the Study

The study adopts the historical and critical methods of philosophical inquiry. The historical method traces how Aquinas's thinking on reason and morality developed and how it relates to earlier and later positions. The critical method examines the strengths and weaknesses of his arguments and of the objections raised against them. The primary text is Aquinas's Summa Theologiae, supported by recent scholarly studies of his natural law and practical reason and by other relevant philosophical and religious literature.

References

Brock, S. L. (2020). The light that binds: A study in Thomas Aquinas's metaphysics of natural law. Pickwick Publications.

Murphy, M. (2025). The natural law tradition in ethics. In E. N. Zalta & U. Nodelman (Eds.), The Stanford encyclopedia of philosophy (Summer 2026 ed.). https://plato.stanford.edu/archives/sum2026/entries/natural-law-ethics/

Vaccarezza, M. S. (2017). Beyond a dichotomy: Aquinas's theory of natural law as a form of autonomous theonomy. Dialegesthai, 19. https://purl.org/mdd/maria-silvia-vaccarezza-01