Aquinas believed that in addition to the natural order which philosophy investigates, there was a supernatural order which did not supersede the natural but was simply beyond the competency of philosophy to understand. Faith, then, did not destroy Reason, rather, it brought it to a higher fulfillment.

In Aquinas, we see this most powerfully in his ethical views. Here, the relationship between natural and supernatural allows Aquinas to prove that there was such a thing as “natural happiness”, the kind that Aristotle wrote about; Christians, however, also wrote of a supernatural happiness of which Aristotle was unaware. Natural happiness, though, remained important as it set the standard of natural laws. It is these natural laws that allow us to obtain natural happiness. This follows, then, that the same is true of supernatural laws and virtues.

Just as we obtain natural laws through a natural process of moral development, we obtain supernatural virtues by the gift of God.

To begin, we must talk about Aquinas’s idea of natural law.

Aquinas, following Aristotle, understands two different kinds of thinking and reasoning. (1) Theoretical reasoning, which aims at simply knowing the truth (such as thinking through the existence of God), and (2) Practical reasoning which aims at an action, such as judging job offers or revealing someone’s secrets. Both of these sets of reasoning start from a set of principles and proceed by way of argument to some conclusion.

In theoretical reasoning, we start first from principles. We then move to a theoretical argument to a theoretical conclusion. Our starting points, our principles, are self-evident; we know them without proof because they cannot be proved. They are the starting points of an argument not the end-all of the argument. These principles guide our attention to theoretical matters whether we pay attention to them or not. This kind of background knowledge is what Aquinas dubs “Habitual Knowledge”.

Practical Reasoning follows the same pattern: from principles, through argument, to conclusions. The difference is that the arguments are not theoretical but practical arguments. Same for conclusions, which means said conclusions are concrete actions one can take. After all, one doesn’t engage in practical reasoning for theoretical conclusions but rather to figure out what to do.

The first principles of practical reasoning? Natural law. These are the universal laws in practical reasoning that function analogous to principles of theoretical reasoning. Natural law is also self-evident as well as always hovering in the background whether we acknowledge them or not; what drives our decisions on practical matters does not vanish simply because we are not aware of its existence. We know it habitually.

In theoretical reasoning, the one principle which Aquinas describes as “the absolute bedrock” is the principle of non-contradiction, or, that two contradictory statement can’t possibly be true. All of theoretical reasoning rest on this basic principle. To Aquinas, anyone who denies this as “opted out” of rational thought entirely.

In practical reasoning too, there is one principle which is the absolute bedrock. That principle is that Good is to be done and pursued and Evil is to be avoided. This in itself does not obviously tell us much; we must first know what is “Good” and what is “Evil” before one can be done and the other avoided.

To Aquinas, there are three types of Good: (1) the kind that we share with every type of creature, that it is good for us to maintain ourselves in existence. (2) There is the type of good that we share with every animal, that it is good for us to reproduce ourselves and care for our offspring. But, there is also the type of good which belongs to humanity and humanity only, namely, that we are the only rational animal. So, it follows that it is also a good that we should exercise our powers of rational thought. Consequently, it follows that we live in society and strive to know God.

These Goods are not independent but rather arranged hierarchically. The Good that belongs to us alone is a higher good than those that we share with other animals and with all creatures. These Goods are also arranged inclusively; meaning, that the Good of reason does not supersede the other two but incorporates them. It is “Good” for us to exercise reason in pursuing the lower-level Goods, caring for our offspring as well as reproduction should be done with reason (etc.). “The human Good is to be in accordance with reason” is how Aquinas puts it. Just because we share certain goods with animals does not mean we must behave as them, it simply means we must use our “higher” Good to follow the lower.

Aquinas’s theory of virtue is closely connected with his theory of Good.

Virtues are traits which enable us to pursue our Good effectively. They are so deeply rooted in our character that they are a sort of second nature. There are several reasons why we need Virtues to obtain the human Good.

The first reason is that we are simply not built to do spontaneously what Reason requires. We, being animals, share with the lower animals the capacity for sub-rational desire. Aquinas calls this the “Sensitive Appetite”. This desire can then come into conflict with reason. So, we need virtues which enable the Sensitive Appetite to conform to reason. Otherwise, our desires would deform our pursuit of Good. The truly good human life is one where the Sensitive Appetite falls in line with human reason.

There are two virtues which allow the Sensitive Appetite to fall in line to reason. These virtues are “Temperance” and “Fortitude”.

In temperance, we desire what reason recognizes as good and reject what it sees as evil. A perfectly temperate person will not desire excessive food and drink, though the Sensitive Appetite may try and force us into excessively eating and drinking. The temperate person, then, only desires as much food and drink as is deemed necessary.

In Fortitude, this virtue allows us to overcome obstacles when they have been identified by reason as obstructions to obtaining the good. Fear and difficulty do not make us veer off track, Reason and not Reaction governs our actions.

To Aquinas, he calls the capacity to recognize rational desire “Intellectual Appetite” or “Will”. Here, the Will does not need any virtue to enable it to desire what Reason presents as Good. Its very nature is to be rational desire. But, Will does need Virtue for a different reason, namely, the good of the collective; my reason and desire are only good for me, so I need something which tells me the good for the collective. This is why Will needs virtue because it enables it to conform what is good to me to the larger good of the community. This is the virtue of Justice.

But, Reason can go astray. Why? Because the human Good is various and multifaceted. What is worth pursuing in one set of circumstance may not be good to pursue in another set of circumstances. We need, then, a virtue within Reason itself which will enable us to navigate these difficulties with insight. This is called “Practical Wisdom” or “Prudence”.

All of these virtues together—Temperance, Fortitude, Justice, and Practical Wisdom— are called the cardinal virtues. Derived from the Latin cardio which means “hinge”, all Good action hinges on these four virtues because they enable Reason to allow us to discern what we ought to do while also enabling desire to fall in line with reason.

Now that the natural has been covered, what about the supernatural? Let’s start here: when Aquinas says that the “life of reason is the human good” does he mean the life of theoretical reason or the life of practical reason? To Aquinas, it is Practical Reason since it is directed at an action, not thinking for its own sake; a life which thinks for its own sake would never act and merely luxuriate for its own needs. Such a life to Aquinas is not human but superhuman. This is the transition point from natural happiness to supernatural happiness.

Yes, the life of contemplation, of theoretical reasoning, is supernatural. According to Christian doctrine, however, God intends human beings for just such a supernatural life. This is the life of Heaven, of a perfect vision of the Truth that is God. We cannot cultivate such a life by cultivating our natural capacities. The life of contemplation is a gift, not an accomplishment.

By insisting on a supernatural destiny for human beings, Aquinas marks himself as different from those “Integral Aristotelians” who asserted the autonomy of the natural order as understood by Aristotle. This idea of a supernatural good is what supersedes Aristotle. But, Aquinas also parts way with conservative thinkers by insisting that the supernatural does not destroy the natural; to Aquinas, the supernatural builds on the natural. In sum, the basic structure is the same, it is merely that the supernatural is a much more developed idea than the natural, the ideas which underpin the natural conception, then, is not wrong, merely underdeveloped, this is where the idea of “Infused Virtues” comes into play which perfects the naturally acquired virtues.

It should be noted, however, that the Infused Virtues function a little differently from their natural counterparts. Infused Temperance, for example, differently regulates the Sensitive Appetite with different goals; Acquired or natural temperance regulates by tempering the Sensitive Appetite for the sake of the natural good as discerned by reason but infused temperance allows us to do the same thing but for God’s own sake instead of Earthly sake.

The relationship between the Acquired Virtues and their Infused counter-parts is complicated; for example, Practical Wisdom, at its most mature form, gives insight into all matters of life itself while Infused Practical Wisdom is wholly concerned with matters about salvation. If one has only Infused Practical Wisdom, then one will be clueless about most things except in obtaining salvation. The same goes for the other virtues which will put persons in a position where they are given gifts from God but do not have the earthly experience in which such virtues are normally tested.

What about the concerns of our supernatural life, however? Do we not need virtues which deal with our divine destiny? Yes, we do, and these are the three theological virtues of Faith, Hope, and Charity; Faith is a virtue in the intellect which enlightens us concerning supernatural truth; Hope and Charity are virtues in the Will that enable us to desire and choose our supernatural good in the right way. These theological virtues are given to us by God, they are infused.

One will notice that the transition traced in this summery is a seamless one. This is because to Aquinas, there is a smooth line of departure from Reason to faith, from philosophy to theology, and from nature to grace. In this way, Aquinas made Aristotle’s philosophy a “partner” in the Christian faith instead of a rival or an alternative. Aquinas, then, integrated Aristotelian thought into Christianity which preserved the integrity of both. An amazing accomplishment which continues to this day among those who are concerned with the project of ‘faith seeking understanding’.

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