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What is natural law?


Benozzo Gozzoli | Public Domain

Fr. Matthew Schneider, LC - published on 07/30/21

The term "natural law" appears frequently in discussions of law and morality. But what does the Catholic Church mean by the phrase?

In discussions on morality and legal issues, we often hear the term “natural law.” However, almost every time I’ve seen it used outside the context of academic Catholic philosophy and theology, I see people misunderstanding it.

Unfortunately, most explanations I’ve seen likely go over many people’s heads. I hope to provide an explanation an average person can understand in three steps: 1) natural, 2) law, and 3) Church teaching. After that, I’ll take up some misunderstandings.

A world of meaning

What do we mean by nature? The term can mean many different things. In this context, we are looking at what makes a thing the type of thing it is.

For example, outside my window I see a pine tree and an oak tree. I can tell each one has a natural unity and is a single thing. I know each is made up of leaves/needles, cells, etc., but works together for the good of the whole. The unity also holds it together as one thing in space (a flock of birds might work for the good of the whole, but we can distinguish separate birds). I could also show genetic unity. I can tell each are tress both by genetics and by looking at them and noticing all of the normal traits of trees. Then I can distinguish which type of tree also by observation: whether by simply looking out the window or by using complex scientific instruments to analyze it.

Now, is that which makes this tree an oak tree unique to this tree, or something shared by all oak trees? I think we’d all agree there is something oak trees have in common. Otherwise, it would be tough for there to be so many instructional guides for growing oaks that all are basically the same. This commonality of all oaks is called the substantial form or the nature of an oak tree. This nature never exists on its own and is always either existing in a particular oak tree or in our mind categorizing all oak trees together. This nature is never directly observable, but we can observe the effects like common genes or common growth patterns.

The same is true for humans. We share a common human nature. There is something common to all that goes beyond sharing 46 chromosomes that are largely the same. Human nature includes our existence as both spiritual and material beings.

Morals need a law

Now, we need to ask what is proper for a thing? What leads it to its goal? What’s proper is based on its nature which is directed towards some goal. For a tree, this is growing big and tall then reproducing.

If we go back to pines and oaks. Both need sunlight, water, etc. But they don’t need exactly the same things. Pines grow great in acidic soil, but oaks do not. The nature of an oak requires that you grow it in a less acidic soil for it to reach its goal: in overly acidic soil it won’t grow tall and strong. Both types of trees also need sunlight and certain nutrients in the soil to grow.

Likewise for humans. We are called to the ultimate end of union with God in eternity, but we are also called to other secondary ends together, usually referred to as “human flourishing.”

For example, we know that man needs to eat to survive, but there is a healthy medium of how much to eat for health (neither anorexic nor excessively obese). We also understand that humans have a certain dignity that means they can’t be owned as slaves, and they have a right to private property so theft is wrong. Such an understanding of natural law also looks at our sexual organs and sees that they are ordered towards sexual acts that are open to procreation and unite spouses. All of these secondary ends are part of human flourishing as we don’t flourish if we are unhealthy, if we lack property, if we are owned by another or if our sexual desires are contrary to reason. Once you get into such details, not everyone agrees on what is proper for human nature.

Church teaching on natural law

The Church has always taught that natural law exists as something imprinted on human nature.

Augustine and Aquinas both clearly indicated that this moral law was impressed into our nature. The Synod of Arles in 473 reaffirmed Augustine and said natural law was “the first grace of God before the coming of Christ.”

In modern times, the Church has reaffirmed this reality multiple times. In Gaudium et Spes (16) of Vatican II, we read: “For man has in his heart a law written by God; to obey it is the very dignity of man; according to it he will be judged.” In Veritatis Splendor (36), John Paul II noted, “The natural moral law has God as its author, and that man, by the use of reason, participates in the eternal law, which it is not for him to establish.”

Incorrect views on natural law

The most common mistake I’ve seen is making false equations: two variations are common.

The first variation assumes that men are just part of nature, so if some animal does X, it’s OK for humans to do so too. This forgets that part of human nature is spiritual / rational, so that this element must be taken into account in order for us to reach our ends properly. Plus humans are social in a way no other animal is, and this implies certain goods like keeping to your spouse for sexual activity to avoid social conflict.

The second variationequates natural law with a law of nature. A law of nature (say, gravity) always happens, while it is physically possible for humans to starve themselves or to steal from others, thus breaking natural law.


Natural law is important to understand both ethics and law.

Clarence Thomas has noted that a universal natural law as understood by Lincoln and Frederick Douglas is what allowed them to fight for abolishing slavery. It was only by understanding natural law that the Nuremburg trials were able to prosecute Nazis, as what they did was legal according to the laws under Hitler. Without some kind of natural law, anything in the legal system is the imposition of the many on the few or the strong on the weak. Sure, almost all want murder to still be illegal, but what about segregation? The many and the strong clearly supported it against the few who objected. However, natural law and certain constitutional interpretation was able to overcome this injustice.

Ethically, natural law is a far more stable basis than what’s socially acceptable or culturally recommended. As humans are the same, morals are the same. Arguing that morals have changed because it’s the 21st century is like saying morals change on Tuesdays (cf. G.K. Chesterton).

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