Have you ever heard of natural law? I’m in my final semester of law school, and have only encountered the term twice in lecture halls, both times resulting in the professor quickly brushing it off as judges looking to the sky and waiting for a “right answer” to come to them.

But that is not the natural law, or at least, not the Catholic understanding of it. Most fully formulated by philosopher and Dominican theologian Thomas Aquinas, the natural law is a law that has been written onto every human’s heart by God, and that every human may access using her capacity to reason. Natural law is significant because it unites all human beings. Being accessible to reason, the natural law transcends cultural differences, and holds true no matter the circumstances. Using natural law, we are able to see the way God intends for us to live, and to know the ultimate good which is, unchanging across time and place.

The natural law is a law that has been written onto every human’s heart by God, and that every human may access using her capacity to reason.

Isn’t that awesome? Let’s dive deeper into natural law and its significance in our lives.

What is natural law?

Again, natural law is based in our God-given ability to “discern by reason the good and the evil, the truth and the lie” (CCC 1954). In other words, it is our capacity to reason out the objective good in any situation.

[Natural law] is our capacity to reason out the objective good in any situation.

Importantly, natural law does not mean waiting for some kind of gut feeling to reveal to us what is right and what is wrong—it is an exercise in reason, a search for an objective answer. The good should be able to be determined by every rational being.

Why should I care?

Paragraph 1959 of the Catechism sums it up nicely: The natural law, the Creator’s very good work, provides the solid foundation on which man can build the structure of moral rules to guide his choices. It also provides the indispensable moral foundation for building the human community. Finally, it provides the necessary basis for the civil law with which it is connected, whether by a reflection that draws conclusions from its principles, or by additions of a positive and juridical nature.

Natural law is always present. It is immutable and undeniable (CCC 1958). It is something we encounter daily, and its principles should form the basis of our civil laws. Basically, it is a framework through which we can use our God-given reason to identify the objective good and objective evil. Therefore, we should work to understand how natural law operates in order to live in accordance with the Truth.

So, how does natural law work?

Catholic philosopher Edward Feser explains natural law according to the following principles:

1). Human beings seek to do what our intellect tells us is good and avoid what our intellect tells us is bad.

This is Thomas Aquinas’ basic tenant of natural law: do good and avoid evil. It’s pretty straightforward. In fact, so straightforward that both Aquinas and Feser refer to it as self-evident: we do what we think will bring us good. Now, what I think is good and what you think is good may be two different things. For example, I may think that it is good to always follow the speed limit, regardless of the situation, while you may think it is good to go over the speed limit if you are late to your grandmother’s funeral. In both instances, we are seeking to do good and working to avoid evil. This can be the case even if a person does something she thinks is bad in order to achieve something else she thinks is good. For example, a bank robber may know that it is bad to steal, but decide that the good that will come to her from the money she steals will outweigh the bad.

Okay, so pretty simple so far. Humans act in ways that they think will result in their good. But, how do we know what is good? That brings us to the second principle of natural law.

2). Human beings have the ability to reason what is objectively good.

Objective good! This is where it’s at. This means that what is “good” and what is “bad” are not relative to our own preferences. “Good” and “bad” exist outside of ourselves.

So, how are we supposed to know whether something is good or bad? Something is good when it attains its natural end. “Natural end” can also be thought of as something’s ultimate purpose (some use word “thriving”). Let’s start with a famous example. A triangle, by definition, is a shape made up of three connected lines with angles that add up to 180 degrees. Though it sounds silly, we can say that a triangle “ultimately thrives,” or reaches its natural end, when it meets this criteria.

If you go to Microsoft Word, select the triangle shape, and “draw” a triangle, you will create a triangle with three straight lines, completely closed, with angles that add up to 180 degrees. This is a “good” triangle--it meets its natural end.

Conversely, if you draw a triangle while sitting on a moving bus, it will not look like the Microsoft Word triangle. The lines will not be straight, they may not close completely, and as a result, the angles will not add up to 180 degrees. Because this shape does not conform to the definition of triangle, it is a bad triangle--it fails to thrive. That is not being mean to the triangle, it is an objective truth.

Yet, looking at this bad triangle, we know that it is still a triangle. Its form is that of a triangle, but it has not achieved its natural (perfect) end.

Another example is that of a three-legged cat. We characterize cats as mammals that have four legs. However, just because this cat does not have four legs, does not mean it is not a cat. It still has the form of a cat, but it has not reached its natural end of cat-ness--it is not ultimately thriving as a cat. Therefore, we could say the cat is not a good cat. Again, this isn’t being mean or saying that the cat has less worth than other cats, it is an objective comparison between this cat and the natural end of what it is to be a cat. Get it? Kind of? Okay, bear with me here.

Looking to human existence, we then ask, what is the natural end of being a human? What leads to our ultimate thriving?

Feser goes through a lot of options, and I’ll quote some of them here (internal citations omitted):

It cannot be wealth, because wealth exists only for the sake of something else which we might acquire with it.

It cannot be honor, because honor accrues to someone only as a consequence of realizing some good, and thus cannot itself be an ultimate good. . . .

Nor can it be power, for power is a means rather than an end and might be used to bring about evil rather than genuine good.

It cannot be pleasure, because pleasure is also a consequence of realizing a good rather than the realization of a good itself; even less likely is it to be bodily pleasure specifically, since the body exists for the sake of the soul, which is immaterial. . . .

But neither can even it be a good of the soul, since the soul, as a created thing, exists for the sake of something else (i.e. that which creates it).

Aquinas concludes that, therefore, our ultimate good is to be found “not in any creature, but in God alone . . . Wherefore God alone can satisfy the will of man . . . God alone constitutes man’s happiness” (Summa Theologica, Part I-II, Question 2, Article 8). Therefore, our end is perfect unity with God, which we will only realize in heaven. Consequently, what enables human flourishing is the same as what will get us to heaven. This is our ultimate good, but we can apply these same principles in other, more specific, situations to arrive at the solution that most accords with our natural end.

What enables human flourishing is the same as what will get us to heaven. This is our ultimate good.

We don’t always know, in a particular situation, how to get to heaven or what will bring us closer to God. And again, the natural law comes in to help us discern. Aquinas states that “all those things to which man has a natural inclination, are naturally apprehended by reason as being good, and consequently as objects of pursuit, and their contraries as evil, and objects of avoidance.”

This sounds kind of confusing. Because doesn’t “natural inclination” sound like instinct or desire? And remember, we’re talking about reason here--not a gut feeling. By “natural inclination,” Aquinas means those activities which nature intends for us. In other words, our “natural inclination” is what we need to do as human beings in order to thrive. And though we may have instincts or desires that align with what we need to do to thrive, we also may not.

For example, in order to thrive, it is true that one must eat food. And, most of us have both the instincts and the desires to eat food. However, those who suffer from anorexia have a desire to avoid consuming food, while those struggling with overeating problems have a desire to eat more food than they need. This example shows why we cannot depend on our instincts and desires alone to tell us what is good, because our instincts and desires may be disordered, that is, not ordered toward our thriving.

We cannot depend on our instincts and desires alone to tell us what is good, because our instincts and desires may be disordered, that is, not ordered toward our thriving.

Consequently, we must take a “third person” perspective on what leads to our thriving. This allows us to detach from our own desires and to view ourselves objectively. (If you like, try looking at what you are doing from God’s perspective.)

Aquinas, Feser, and other philosophers have much more to say on determining the good and the bad, but I’m going to leave it here for now since this is just a basic introduction. Suffice to say, it is not always clear what the good and the bad are, but by using our reason and focusing on what leads to ultimate union with God—physically, mentally, and spiritually—we should be able to determine the good in a situation, at least most of the time.

With that, let’s bring it all together.

3). A rational person will figure out what is the objective good and direct her actions to that end.

So, if it is self-evident that human beings do good and avoid evil, and if human beings have the ability, through reason, to determine objective good and objective evil, then, human beings ought to do what is objectively good and avoid what is objectively bad! And that’s the natural law.

By understanding natural law, we are able to better grasp God’s desire for our lives and live consistently, no longer slaves to our desires. We can use our reason to look at situations objectively and choose the good. We can get that much closer to becoming saints!