Moral life according to Aquinas requires not just our own self-cultivation (development of virtues exercised in ways appropriate to our state of life), but also external helps. These external helps are not external in the sense of being wholly outside us (and indeed, the most important cases are not), but in the sense that they are not things we ourselves develop; rather, we receive them in some way. There are many such helps; the most important are law and grace. Both of these will also in a number of ways overlap and support the virtues.
Altarpiece by Carlo Crivelli, showing Thomas Aquinas's association with law and grace in both sacrament (church) and teaching (book).
One of Aquinas's most famous philosophical contributions is his definition of law. According to Aquinas, law is
(1) a rational ordering
(2) to common good
(3) by one who is caretaker for what is common
The purpose of a law is to act as guide and be a standard for rational beings, so it is necessary for it to be something that reason can use as a guide and standard. There are many kinds of rational guides and standards, though, even in practical matters. To be a law, it must be something that applies stably to everyone in the community; it must therefore deal with some stable motivation for the community. As an Aristotelian, of course, Aquinas takes this to be happiness, but it must be happiness insofar as it is shared as a goal by the community itself. Thus all law is concerned with that kind of happiness that is the totality of good in a life insofar as it is lived with others. Every community is formed around some common good, some good shared in common by the members of that community, whether that good is possessed or sought. Indeed, community just means 'the whole of what is common' or 'the state of having things in common'. This brings us to the third element of law; to have something of the authority that comes from the common good constituting a community, the law has to be put forward in a way adequate to that common good, that is to say, it can't be just anybody's rational ordering, but must be the rational ordering of a reason that is particularly connected to the common good. It must be put forward by someone who has the care (cura) for what is common. Usually the natural caretaker for a community is the whole community itself working together, but in practice it is often somebody who is designated a public person by the community and who acts as viceregent of everybody. This feature is what distinguishes law from the advice of private persons (which may be providing a rule relevant to common good but are not acting on behalf of the community whose common good it is) or the rules imposed by parents on a household (parents having the care of what is by definition only a partial contribution to common good). And finally, it must be actually applied. That is to say, if the rule is to be obeyed, it must be such that those who obey it can know it, so it must be promulgated or published.
Aquinas takes this to be a strict definition of law. Every law will have all four properties, anything that has all four properties is a law, and these are both true regardless of whether we call it a law or not. Both of them have points that are controverted by other philosophies of law. For instance, it follows from Aquinas's account that something cannot be a law at all if it is in itself inconsistent with common good -- such a thing is really a usurpation of power and an act of violence against the community, even if people call it a 'law'. And there will be quite a few different kinds of laws that will fit this definition, even if they are not what we typically call 'laws'.
The most general and encompassing kind of law is what Aquinas calls eternal law, which is the rational order of divine providence; God, being the good to which the totality of the world is directed and the source of everything in the universe, is the natural caretaker for what is common to the entire universe. The eternal law is in a sense the idea of law itself insofar as the entire world is subject to it. All other kinds of law depend on this, for the simple fact that none of the other kinds of law would exist without it.
More directly relevant to any consideration of ethics and reasoning, however, is natural law. Human practical reason operates according to practical principles, which are the standard against which one measures whether a plan or decision is a good one or a bad one, whether it 'makes sense', whether it is worth implementing, just like the principle of noncontradiction and other principles of theoretical reason are standards against which one measures whether theoretical reasoning is good or bad. These practical principles cover absolutely every practical plan and decision we make. And the most general of these, the noncontradiction of practical reasoning, so to speak, is Good is to be done and sought, bad is to be avoided. If any practical plan violates this, then to that extent it is irrational. To be rational is in part to recognize and apply principles like this.
This principle of practical reason concerns every kind of good or bad. But suppose we look in particular at good that human beings share as human beings. There is a human community; we can recognize goods that we have in common with all other human beings. Examples are things like survival, reproduction and education and care for children, or rational life. But if we focus on this kind of good, then we find that the first principle of practical reason fits the definition of law. By supposition it is being applied in a way that concerns common good; by definition it is a rational ordering to good; as all rational beings can know it, it is promulgated; and it is promulgated by all human beings, who are together the natural caretaker of the common good of all human beings. The same will be true of other principles insofar as they touch on human common good, as well as any conclusions of those principles that do the same. There is a law natural to our own reason, which is our participation in eternal law, and its existence follows from the bare fact that we are rational beings who share a rational human nature with each other. We are always already under law.
An important aspect of how Aquinas builds his account of natural law -- and it is sometimes forgotten even by natural law theorists -- is that natural law serves as a general rational standard, not a rigorous account of the full moral life. It is in fact very difficult, and sometimes impossible, to reason out precisely what natural law requires for every single situation. Natural law is linked to acquired virtue in that it obligates us to acquire each virtue and act according to it in some way. It does not obligate us to every act of every virtue, nor does natural law cover every aspect of moral life; but as our virtues all have bearing on the common good of the human race, virtues in a general way are all obligatory under natural law. Natural law cannot replace the virtue of prudence, although it does obligate us to be prudent, and most of the reasoning involved in our actual moral lives will derive from the virtue of prudence approximating and estimating and developing guidelines appropriate to circumstances, not directly by reasoning from natural law.
It is also perhaps worth recognizing that all obligation is in Aquinas's account communal. Without a common good, there is no law. Without a role in a community, there are no offices, i.e., duties based on our state of life. Private goods, goods belonging entirely to one and only one individual, obligate nobody. Obligation is something that arises out of our lives in community. But it is also important to recognize that we are all in community, and cannot function as human beings without being so. Reason is in-common by its very nature; we are social beings not merely in the fact that our natures lead us to try to be in society but in the fact that we are already in a community by the bare fact of being human. Natural law reflects this.
Natural law is a general standard; we often need something more specific. This gives us positive laws, laws that are made for specific purposes, of which there are two kinds, divine law and human law. The essential idea in both is that our moral life is a life of virtue, and acquired virtue is something we can have only by training, for which natural law provides only general guidance. Some of this is training we ourselves develop. But none of us are sufficient of ourselves to determine all the training that must be done in order to be virtuous. We need in part to be trained by others. Advice and admonition will sometimes be enough, for at least some things, but their implementation still depends entirely on our choosing to take them into account, and in matters of common good, it's not possible for everybody just to make their own decision about what should be done, particularly given that some people will choose in ways that are utterly inconsistent with virtue. So we need additional laws that give us specific guidance going beyond natural law. If this comes from God, like the Decalogue (Ten Commandments) or the precepts of Christ, it is divine law; if it comes from the human community, it is human law. Human law has no legitimate rationale other than to make it easier for us to become and to be virtuous people living with others who are becoming and being virtuous people.
All genuine human law by definition has to be consistent with natural law. However, because human means of enforcement cannot cover everything practical reason can, not every precept of natural law can be legislated by human law. For instance, if in addition to recognizing that lying is against natural law, we made it illegal ever to lie by human law, the enforcement of this would be so difficult that we certainly could not enforce it without violating natural law ourselves. So it makes sense simply to endure the fact that people will violate natural law by lying, and confine ourselves only to those things we can morally enforce. This is called the doctrine of toleration: human law must sometimes tolerate violations of natural law. On the other side, though, it is often necessary for us to be in agreement about particular matters that natural law does not on its own decide, so human law gives us a way to work together on particulars that are not rationally necessary but have to be determined if we are to act as we should.
Besides law, the other major factor of our moral lives that enters from outside is grace. 'Grace' is used in three different ways, all connected with the root meaning of the word in Latin, which is freedom. It can mean the love or favor of another person (this is the sense in which you are in someone's 'good graces'). It can mean a gift that is freely given. And it can mean a response to such a gift that is appropriate (this is the sense in which we are grateful or gracious). These are all interlinked: the first gives rise to the second, which gives rise to the third. The second, however, is the primary kind of thing Aquinas has in mind, and the most important thing fitting this description is the quality of the soul, which makes us worthy of God, added to our very being by God in order to lead human beings to God. Such grace can either be a gift directly leading someone to God (sanctifying grace), or a gift helping someone to lead someone else to God (gratuitous grace).
Grace, of course, is the foundation of the infused virtues, which raise human beings to a higher degree of society. However, Aquinas also thinks that we cannot fulfill the obligations of natural law without grace -- we have a tendency toward wrongdoing, so we need grace to compensate for this, and even if we did not, we would still need grace in order not just to conform to the law but to do so in a way that is appropriate to it. We cannot fully act well without divine help; our freedom needs a free gift from God in order to come to the end suitable for it.
In practical life, the notion of grace is generally (although not always) linked with the notion of sacrament, which is also a point at which law and grace intermingle. A sacrament is a sensible sign of something holy insofar as it contributes to our being holy, like sacrifices and tithes. Prior to any specification by divine law, in a regime of natural law, our sacraments our determined by our own vows or (at times) by special divine inspiration; these sacraments have only the most indirect connection with grace, in that we hope that God will give grace to help us because we are recognizing explicitly that we need it. When God gives the Law through Moses, however, He institutes a new regime, one that both gives a divine specification of the sacraments and a greater assurance of divine grace. These sacraments, like circumcision or Old Testament sacrifices, signify grace more consistently and accurately than other sacraments could, and convey a sort of promise that grace will be given. However, as a Christian, of course, Thomas holds that these sacraments are merely preparatory to a more complete form of sacrament, given in the New Law received from Christ; this New Law is itself given by grace, and its sacraments, like baptism, are signs of grace that both contain and cause grace directly to those who are properly disposed. It is in the sacrament of baptism, for instance, that we receive the infused virtues, and it is by the sacrament of penance that the infused virtues are restored if we act so as to lose them. And since, of course, law is by its very nature a communal thing, and determines the nature of sacrament, our primary form of expressing our readiness for grace, and in the case of the New Law, the primary form of receiving it, grace, like law, makes clear that our moral life is a communal life.
Both law and grace, Aquinas thinks, are ultimately founded on divine providence, and thus the moral life can only be fully lived when it is lived in a way appropriate to the broader context established by this divine providence, which is like a higher prudence -- 'prudence', in fact, is just a short form of the word 'providence'. We are not isolated individuals, and cannot be moral as isolated individuals. Our moral life presupposes others, whether by law or by grace or by some lesser assistances like these, and it is done in community with others, both God and other human beings, and it is diversified according to the communities of which we are part and our roles in those communities, and the goal of that life, whether the incomplete goal of peaceful and harmonious felicity or the higher goal of beatitude, is a goal shared in common. And understanding that Thomas sees life as structured in this way by community is essential to understanding his virtue ethics.