What we mean by "nature" vs. what people think we mean

I know when we used the term “nature” when discussing morals, it means something different from “existing in the great outdoors”.
So when one person says _____ is unnatural (usually sexual) someone always comes along and points out “well animals do _____! appealing to nature is a fallacy!” But it seems that they themselves commit a fallacy of equivocation.
How can I articulate the difference, though?

You’re talking about human nature - what is natural to humans. Just because some creatures (such as many fish) eat their young doesn’t mean it would be natural for a human to do this.

Morals are confined to human nature, because only humans have them. A fish does not sin by eating its children. A raccoon does not sin by dumping your garbage. A dog does not sin by taking a bone from a smaller dog. They are all acting according to their natures, but they have no morals (so they have no sense of right and wrong). One cannot impose their natures onto ours to justify behavior we know to be wrong.

I like this.

See the Catechism ( linked below ), Part III, Article 1, para 1950-1960. Sexuality is dealt with in Article 6. Article 1 explains the moral law as a subset of natural law.


When we say nature, we mean how something is supposed to function. As a quick example, suppose I flex my arm 90 degrees. That’s a natural movement, it’s what’s expected. On the other hand, suppose my arm was extended 90 degrees beyond straight. You’d say that it was broken (perhaps when you called 911 about the man with the bone protruding from his arm)–meaning there’s a way it’s “supposed” to function, and thus a correct way and an incorrect way for it to function. Now, animals break their bones very often, sometimes horrifically, but that’s not how they are supposed to function, so we say extreme hyper-extension is unnatural for the human arm.

Similarly, it’s quite clear how the reproductive system is supposed to function. It necessarily involves a male and female. Anything else is unnatural. It’s a broken behavior. It carries no benefits and many risks. Even homosexual couples who claim to have kids always require a donor sperm or egg-- even if it’s heterosexual by proxy, reproduction still requires heterosexuality. Now, if the reproductive system necessarily requires a male and a female, this would logically suggest that sexual unions should only be between a male and a female.

Suppose humanity evolved arms that could bend 90 degrees beyond straight. Would the movement still be unnatural?

If it became common to our species it would be, by definition, natural.

Likewise, if humans evolved to be fully intersexually androgynous then the idea of “male” and “female” would no longer be natural to our species, and anyone would be free to marry anyone else without moral repercussions.

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