Aquinas's Summa Theologica: The Five Ways and Natural Law

Suppose someone asks whether a law can be properly enacted and still be unjust. That question distinguishes the existence of a rule from the reasons for obeying it. Aquinas develops an account of law in which human enactment must be assessed against a rational standard of the good. Elsewhere in the same work, he asks whether reason can establish that God exists. Both inquiries concern what reason can discover, but they require different arguments.

The Summa Theologica, also called the Summa Theologiae, treats these questions within a larger theological project. Its opening questions investigate sacred teaching and the existence and nature of God. The discussion of law appears much later, in the First Part of the Second Part. Reading the Five Ways and natural law together helps explain how Aquinas connects reasoning about created things with reasoning about human conduct, without collapsing ethics into a single proof of God.

A typical article first presents objections to the position Aquinas will defend. A short opposing statement follows, then his own answer and replies to the objections. The sequence matters because an opening objection can contain a proposition that the final answer rejects. Quoting an objection as Aquinas's settled view reverses the purpose of the format.

Part I, question 2, illustrates the method. Its first article asks whether God's existence is self-evident. An objection proposes a route from understanding the word God to recognizing real existence. Aquinas's reply challenges that route: understanding what a word signifies does not establish that its subject exists outside the mind (I q.2 a.1, reply 2).

His own answer distinguishes what is self-evident in itself from what is self-evident to us. Because we do not know the divine essence, he argues, we must proceed from effects more familiar to us. This describes the direction of the proposed inquiry. It does not treat the conclusion as obvious to anyone who understands the vocabulary.

Article 2 then distinguishes demonstration from a cause from demonstration through an effect. Aquinas argues that an effect can establish that its cause exists even when it does not supply a complete understanding of that cause. He applies this claim to natural knowledge of God. The Five Ways in article 3 are intended to implement reasoning from effects.

Each Way begins from some feature Aquinas finds in the world and reasons toward a first principle. They are offered as demonstrations, rather than as five measurements whose results can be averaged. Assessing them requires examining their premises about change, causation, necessity, degrees of perfection, and natural ends.

Aquinas names the conclusion of each argument God. The compressed arguments do not, by themselves, establish every attribute discussed later in the Summa. The subsequent questions investigate such matters as simplicity and goodness. Treating the Five Ways as the complete argument for every Christian doctrine would erase that further work.

The opposite simplification is also misleading. Aquinas does not think reason can say nothing about God beyond an unidentified first cause. His project includes further reasoning about divine attributes, while recognizing limits to natural knowledge. The distinction concerns what particular arguments establish and how later claims are supported.

The first Way begins from motion, understood through the transition from potentiality to actuality. Potentiality is a capacity to be otherwise; actuality is the relevant condition being realized. Aquinas's example concerns fire heating wood. The wood can become hot, and something already actual in the relevant respect brings about the change (I q.2 a.3).

The qualification concerning respect matters. Aquinas does not say that a thing cannot be actual in one way and potential in another. Wood can already be warm while capable of becoming warmer. His principle denies that the same thing is actual and merely potential in precisely the same respect at the same time.

He then argues that whatever is moved requires a mover, and that a series of movers cannot proceed without a first source. Otherwise, on his account, the intermediate movers would not exercise the derivative activity through which change occurs. He concludes that there must be a first mover that is not itself moved.

This is not simply the claim that every object needs an initial shove at some date in the distant past. The text's explanatory framework concerns the dependence of movers, including an example of a staff moved by a hand. Reading the Way solely as a claim about the earliest event misses the role of that dependence.

A critic can question whether every instance of change requires the actuality Aquinas specifies, or whether the regress principle excludes the relevant infinite series. Naming these commitments makes the disagreement more precise than merely announcing that motion exists. The observation initiates the argument; it does not by itself establish the first mover.

The second Way concerns efficient causes, causes that bring about effects. Aquinas denies that something can be its own efficient cause, since it would then have to be prior to itself in the relevant causal order. He argues that an ordered series requires a first cause if intermediate and ultimate effects are to occur.

He does not begin with the unrestricted premise that everything has a cause, then arbitrarily exempt God from it. The stated principle concerns the ordered efficient causes encountered in experience and their dependence. A first cause is introduced as what accounts for the derivative causal order, rather than as one more item required to have the same dependence.

Whether this succeeds remains a philosophical question. We can ask what establishes that the series has the structure assumed and why an infinite derivative order cannot explain its members. Saying that an infinite regress is forbidden without examining the kind of regress would leave the central premise undefended.

The first and second Ways overlap in their concern with dependence, but their vocabulary differs. The first develops the actualization of potentiality; the second focuses on efficient causation. Keeping those starting points distinct makes it easier to see what each inference needs.

The third Way begins with things that come into being and pass away. Aquinas calls them possible to be and not to be. He reasons that if everything were of this kind, there would have been a time when nothing existed. If nothing existed then, he argues, nothing could have begun to exist, which conflicts with the existence of things now.

This temporal step deserves scrutiny. From the fact that each member can be absent, it does not follow simply that all members are absent together. Imagine time extending indefinitely into both past and future, with lamps numbered by all integers, without a first or last. Each lamp numbered n is lit from time n until just before time n + 2. At every time at least one lamp is lit, although every individual lamp has times before and after its lit interval when it is dark. This idealized model challenges only the move from individual absence to universal absence. It does not establish a physically possible universe or disprove Aquinas's whole metaphysics.

The chapter should therefore preserve the argument's actual step rather than replacing it with the different claim that the universe needs an explanation. A modern reconstruction may defend a related dependence argument, but it must disclose the change. Understanding a historical argument includes identifying where its wording invites an objection.

Aquinas then distinguishes necessary beings whose necessity has a cause from a being whose necessity does not derive from another. He argues against an infinite regress of caused necessity and concludes that there is a source necessary through itself. This additional stage is needed because reaching something necessary is not yet, in his reasoning, reaching the ultimate source.

The fourth Way begins with degrees of goodness, truth, and nobility. Aquinas reasons from these comparisons toward a maximum and then toward a cause of being and perfection. The premises concern the relation between degrees, their maximum, and causal explanation; observing comparative language alone does not establish them.

For a hypothetical comparison, knowing that one runner is faster than another does not identify an existing fastest conceivable runner. The illustration shows why a general inference from ranking to an existing maximum would be inadequate. A defense of Aquinas must explain why the perfections he considers support a stronger connection, rather than treating any ranked quantity as sufficient.

The fifth Way concerns natural things without intelligence that regularly act toward ends. Aquinas compares their direction to an arrow directed by an archer and infers an intelligent director. An end here is what an activity is directed toward; it need not be a purpose consciously entertained by the thing acting.

The argument is therefore broader than an observation that one organ looks complicated. Its premise concerns regular directed activity in nature. A critic can ask why that regularity requires intelligence, while a defender must explain the proposed dependence. Neither repeating the archer comparison nor reporting a natural regularity settles that question on its own.

The Ways can thus be studied without declaring them either automatic successes or obsolete curiosities. Each contains an observation and a proposed explanatory principle. The point at which observation becomes explanation is where careful evaluation begins.

Question 13 considers how words drawn from creatures can apply to God. If goodness meant exactly the same thing in both applications, Aquinas argues, our language would fail to respect the difference between creator and creatures. If it meant something wholly unrelated, reasoning from creatures would lose its connection with what is said of God (I q.13 a.5).

He develops an analogical account. The meanings are related through the dependence of created perfections on their cause, without making divine goodness an instance of a creaturely quality in the same limited manner. Analogy here is a claim about an ordered relation in meaning, not permission to substitute any attractive metaphor.

This position depends on the metaphysical relationship Aquinas claims between creatures and God. It cannot be established just by choosing the label analogy. Its explanatory task is to show how language can convey something true while leaving the manner of divine existence beyond the mode in which we encounter creaturely perfections.

In I-II, question 90, Aquinas defines law through reason, the common good, appropriate authority, and promulgation. The account does not identify law with any command issued by a powerful person. It asks who is responsible for the community and how a rule becomes publicly available as a guide for action.

Question 91 distinguishes eternal, natural, human, and divine law. Eternal law concerns divine governance. Natural law is the rational creature's participation in that governance (I-II q.91 a.2). Human law supplies determinations appropriate to human communities, while divine law directs human beings through revelation.

Participation needs explanation here. A rational creature does not merely undergo an external regularity. Human beings can deliberate about what they should pursue and can fail to act in accordance with that judgment. Natural law names practical principles available to reason, within Aquinas's theological account of why those principles have authority.

The framework consequently connects theology and ethics without requiring a separate revelation for every ordinary moral judgment. Yet it does not follow that every person recognizes every moral conclusion equally well. Question 94 explicitly distinguishes common principles from more particular conclusions and their variable application.

Aquinas's first practical principle directs us to pursue good and avoid evil (I-II q.94 a.2). It does not by itself supply a list of detailed statutes. Further reasoning concerns the goods that practical reason apprehends in relation to human inclinations.

His account identifies preservation of life, matters shared with other animals such as generation and education of offspring, and activities belonging to rational social life, including seeking truth about God and living with others. These are ordered sources of practical precepts, rather than a report that every desire is morally authoritative.

The distinction is explicit in question 94, article 4, reply 3: inclinations of the other powers should be directed according to reason. An impulse cannot justify itself merely by being natural in the sense of occurring spontaneously. The theory asks what contributes to a rational human life, which requires assessing desires as well as acknowledging them.

A reader can challenge the proposed account of human nature or the move from its inclinations to obligations. Those challenges concern substantive premises in the theory. They are stronger when they address Aquinas's practical reasoning than when they assume that he simply derives a moral rule from any observed behavior.

Question 94, article 4, distinguishes general principles from particular conclusions in contingent circumstances. Aquinas's example concerns returning entrusted goods. Returning them is generally right, but he considers a case in which someone demands them for use against the community. The circumstances can make what is generally required unreasonable in that case.

The example shows why knowing a general precept does not eliminate deliberation. One must understand what the action would do in its circumstances. This does not mean that a convenient exception cancels every inconvenient obligation; it means that a practical conclusion needs the conditions that make it appropriate.

The discussion also separates whether a conclusion is right from whether people recognize it. Aquinas allows error through such influences as passion and bad habit. Common rational principles therefore do not entail uniform agreement about every detailed moral question.

These distinctions matter when applying the theory to law. A statute may require a common determination among several reasonable alternatives. Another statute may conflict with the good it should serve. Treating every legal disagreement as an obvious violation of natural law would ignore the need to justify the particular conclusion.

In I-II, question 96, article 4, Aquinas says unjust laws can be acts of violence rather than laws in the full sense. He discusses injustice through a law's end, its author, or the unequal distribution of burdens. This is a moral assessment of authority and obligation, not merely a prediction about what a court will enforce.

He also qualifies the consequences. Laws contrary to human good ordinarily do not bind conscience, though compliance may be required to avoid scandal or disturbance. Laws contrary to divine good must not be obeyed. Saying simply that every unjust law may always be disregarded would omit these conditions.

A contemporary hypothetical case can use the distinction without claiming Aquinas judged it himself. Imagine a rule that distributes a community burden solely to enrich its ruler. Asking whether it was enacted through the proper procedure would not exhaust the moral question. One would also need to examine its end, the authority involved, and the distribution of the burden.

This example does not authorize a legal conclusion about a particular modern statute. It illustrates the philosophical difference between an enactment's procedural status and its claim to bind conscience. Applying that difference responsibly requires reasons about the actual circumstances, including the consequences of disobedience.

Read I q.2 in full, including the objections and replies, before reducing the Five Ways to slogans. I q.13 a.5 supplies the account of analogy. For law, read I-II qq.90-91 and q.94, then q.96 a.4 for the qualifications concerning unjust law. This sequence distinguishes reason's proposed starting points from the conclusions it still has to earn.

• Summa Theologiae, I q.2, on God's existence and the Five Ways.
• I q.13, on names and analogical predication.
• I-II q.90 and q.91, on the definition and kinds of law.
• I-II q.94, on natural law and practical judgment.
• I-II q.96, especially article 4, on the obligation of human law.