Once, around ten years ago, someone in an online discussion said that they were raised as a young-earth creationist, but had become curious about the scientific account of the natural world and how it worked. They wanted to know what the problems with young-earth creationism were, from a scientific perspective.

I offered them the best answer I could manage. Here it is:

The first thing you have to do is set aside God as an explanation. Sure, if you assume there’s an omnipotent, omniscient, supernatural creator, then you can explain anything by saying, “It’s God’s will.” The problem is, that doesn’t really explain anything. It doesn’t tell you anything that you didn’t already know.

Take infectious disease, for example. It’s been a scourge of the human race since prehistory. Saying, “It’s God’s will,” or “the will of the gods,” for thousands of years didn’t help people discover the causes or cures for diseases. What did help was examining the natural world with a curious mind, and seeking to understand what was found.

Curious people found out that there are tiny organisms, too small to see, and they’re everywhere. Some of them can colonize our bodies, and some of those cause diseases. There are also chemicals we can use to counter those invading organisms, and those chemicals help us defend ourselves against many diseases.

We didn’t learn those things from saying “disease is God’s will”—regardless of whether it is or isn’t. We learned them from looking at the evidence that the natural world gives us and reasoning about what we found.

So the first thing you do is set God aside. Then you collect evidence and see where it leads you. You try to find natural explanations for the evidence that you find.

So far, that approach has been spectacularly successful.

From that perspective, there are some serious problems with young-earth creationism.

For example, living things are made of lots of carbon, hydrogen, oxygen, and nitrogen, and smaller amounts of other things. There’s more than one kind of carbon, each identified by an atomic number. You can find both carbon 12 and carbon 14 in living things.

It turns out that carbon 14 is radioactive, which means that it falls apart, turning into atoms that aren’t carbon 14 anymore. This happens at a predictable rate: if you keep a lump of carbon 14 around for 5730 years, you’ll find out that half of it isn’t carbon 14 anymore; it’s turned into carbon 12.

So that should mean that eventually all the carbon 14 should disappear, right? It should all turn into carbon 12. Except that radiation from space striking the earth’s upper atmosphere keeps making more of it at a pretty steady rate. It rains down and mixes into the atmosphere and gets carried all over the world.

One kind of carbon is as good as another to living things, so plants absorb it, animals get it by eating the plants or each other, and it ends up in everything living.

This, too, happens at a steady, predictable rate: out of every trillion carbon atoms in a living creature, one of them is carbon 14.

But remember: carbon 14 is radioactive. It falls apart. After 5730 years, only one atom in two trillion will be carbon 14, unless the living thing keeps adding more.

Well, when creatures are alive, they do keep adding more, by consuming it from their environment. But when they die, they stop adding carbon 14. The carbon 14 doesn’t stop falling apart, though; it keeps on turning into carbon 12. The longer the corpse lies there, the more of it falls apart at that steady, predictable rate. So if you measure the carbon 14 in a corpse, you can use simple arithmetic to figure out how long ago it died.

When you do that with a lot of dead things, you find out that some of them died more than 6000 years ago. In fact, a whole lot of them died way more than 6000 years ago.

This isn’t the only way to find out how old things are, by the way. You can measure age by the radioactive decay of elements besides carbon. You can use other natural processes that happen at steady rates, like the deposition of silt, or the changes in magnetism recorded in rocks, or the rate that minerals trap ions the solar wind and cosmic rays. When you use several different measures of age and get close to the same result, you can be pretty confident that you know the age of the thing you’re measuring.

We’ve collected age measurements for a lot of things at this point. We have tons of animal and plant remains that are millions—or in some cases billions—of years old. We have minerals that are millions or billions of years old. We have astronomical evidence of things in space that are millions or billions of years old.

So now our naturalistic picture of the world needs to account for the fact that we’ve found all this evidence of very old things. For example, we need to account for living things that died a lot more than 6000 years ago.

Now, we could just say that God created the world with all that evidence already in it. That’s cheating, though. Remember: the rules are that we look for natural explanations. That’s science. If we resort to supernatural explanations then we’re doing theology, not science.

Also, we’d have to wonder why the creator planted gobs of evidence that the world is billions of years old, if he wanted us to believe that it isn’t, but that’s not science, either

The simplest natural explanation for dead bodies more than 6000 years old is that the earth is more than 6000 years old.

If you run through the mountains of evidence that scientists have accumulated over the past five hundred years, you’ll notice a lot more situations like this, situations where you either say, “Well, God just made the world like that,” or else you try to think of a natural explanation. The natural explanations tend to contradict young-earth creationism.

The earth is more than 6000 years old because we can find things in it that have been around longer than that. Living things evolved from other, earlier living things because we’ve seen it happen in our present environment, and because we’ve found many remains of living things that are different from anything currently living, and because we’ve found sequences of genetic material from which we can infer ancestral relationships.

Human beings, for example, are apes because we have the skeletal structure of apes, the blood types of apes, the reproductive biology of apes, and the genetic makeup of apes. We know humans are apes in exactly the same way that we know chimpanzees are apes.

We can measure how similar or different two pieces of genetic material are, and from doing that we know that humans are closely related to chimpanzees and bonobos. We also know that there used to be other human-like species that are gone now, because we’ve found their tools and their remains.

We know that there were stars in the sky more than 6000 years ago because we know how fast light travels and we can measure the distances to the stars pretty well. Some stars are so far away that it’s taken more than 6000 years for the light to reach us. For example, the light from the Andromeda galaxy—the nearest galaxy to our own—takes two and half million years to reach us.

You can still believe in young-earth creationism, if you want to. You just have to also believe that, for some reason, the creator planted evidence against it all over the place.

But if you want a natural explanation of what we see around us, then young-earth creationism just doesn’t do the job.