Logical Fallacies

Logic can be a valuable tool for Christians to defend their faith. But mistakes in reasoning can derail even the best argument.

I once did a telescope session with a small group of people, including a four-year-old boy who was particularly interested in astronomy. I asked this young budding astronomer if he believed in alien spaceships. “Of course,” he said. I then asked him why he believed in alien spaceships. I’ll never forget his clever response: “How else would the aliens get here?” Pretty logical isn’t it? The aliens would never be able to get to earth without a spaceship. So, clearly, there must be alien spaceships!

This is a wonderful example of a very common error in reasoning—the fallacy of begging the question. This fallacy is committed when a person merely assumes what he or she is attempting to prove or when the premise of an argument actually depends upon its conclusion. In this case, our young student was attempting to prove the existence of alien spacecraft by taking it for granted that aliens have traveled to earth. But that is essentially the point in question. This young aspiring astronomer was reasoning in a circle.

Of course, we expect such humorous reasoning from a four-year-old. As we grow up, we are expected to become rational and not make these kinds of logical mistakes. That’s why it is so disturbing to find that many adults commit the fallacy of begging the question in debates on origins. Some examples are obvious: “Evolution must be true because it is a fact.” But, more commonly, the fallacy is much more subtle. Consider some of the following arguments.

“The Bible cannot be true because it contains miracles. And miracles would violate the laws of nature!”

Yes, miracles can potentially involve a temporary suspension of the laws of nature (not that all of them necessarily do).1 Since the Bible makes it clear that God is beyond natural laws, He can suspend/violate them if He wishes to. But the critic’s argument has simply taken it for granted that violations of the laws of nature are impossible. In other words, the arguer has already assumed that the Bible is false—in order to argue that the Bible is false. He has begged the question.

You may have heard people argue:

“The Bible cannot be true because it teaches that the earth is only thousands of years old; whereas, we know the earth is billions of years old.”

All such arguments commit the fallacy of begging the question. Here is why. Old earth arguments are all based on the assumptions of naturalism (nature is all that there is) and a large degree of uniformitarianism (present rates and processes are representative of past rates and processes). Then, by extrapolating from present rates of various earth processes, the person estimates how long it would take to build up or erode certain geological features or how long it would take for a radioisotope to decay.

But the Bible denies naturalism and uniformitarianism (e.g., erosion rates during the global flood). By assuming naturalism and uniformitarianism, the critic has already merely assumed that the Bible is wrong. He then uses this assumption to conclude that the Bible is wrong. His reasoning is circular.

“Creation cannot be true because you would have to ignore all that scientific evidence.”

But this argument begs the question because it presupposes that scientific evidence somehow provides support for evolution, which has not been demonstrated.

“It makes no sense to deny evolution; it is a well-established fact of nature.”

This argument also begs the question since the truth status of evolution is the very question at issue.

Christians are not always above circular reasoning either. Some have argued,

“The Bible must be the Word of God because it says it is. And what it says must be true, since God cannot lie.”

Of course, it is quite true that the Bible does claim to be the Word of God, and it is also true that God does not lie. But when one of these statements is used as the sole support for the other, the argument commits the fallacy of begging the question. The same line of argumentation could be used to “prove” the Koran, which of course we would deny.

Now, it’s time to get a little philosophically deep. Brace yourself. Begging the question is a very strange fallacy because it is actually valid. Recall that a valid argument is one in which the conclusion does follow from the premises. Normally fallacies are not valid; the fact that their conclusion does not follow from the premise(s) is what makes them fallacies. But, oddly, with begging the question the conclusion does follow from the premise (because it is simply a restatement of the premise). So, the argument, “Evolution must be true because it is a fact,” is valid. But if it is valid, then why is it considered a fallacy?

The answer would seem to be that begging the question is a fallacy because it is arbitrary. Circular arguments of this kind are not useful because anyone who denies the conclusion would also deny the premise (since the conclusion is essentially the same as the premise). So, the argument, “Evolution must be true because it is a fact,” while technically valid, is fallacious because the arguer has merely assumed what he is trying to prove. Arbitrary assumptions are not to be used in logical reasoning because we could equally well assume the exact opposite. It would be just as legitimate to argue, “Evolution cannot be true because it is false.”

It should also be noted that there are certain special cases where circular reasoning is unavoidable and not necessarily fallacious. Remember that begging the question is not invalid; it is considered fallacious because it is arbitrary. But what if it were not arbitrary? There are some situations where the conclusion of an argument must be assumed at the outset, but is not arbitrary.2 Here is an example:

  1. Without laws of logic, we could not make an argument.
  2. We can make an argument.
  3. Therefore, there must be laws of logic.

This argument is perfectly reasonable, and valid. But it is subtly circular. This argument is using a law of logic called modus tollens to prove that there are laws of logic. So, we have tacitly assumed what we are trying to prove. But it is absolutely unavoidable in this case. We must use laws of logic to prove anything—even the existence of laws of logic.

However, the above argument is not arbitrary. We do have a good reason for assuming laws of logic, since without them we couldn’t prove anything. And perhaps most significantly, anyone attempting to disprove the existence of laws of logic would have to first assume that laws of logic do exist in order to make the argument. He would refute himself.

Most of the examples of circular reasoning used by evolutionists are of the fallacious begging-the-question variety—they are arbitrary. Consider the evolutionist who argues:

“The Bible cannot be correct because it says that stars were created in a single day; but we now know that it takes millions of years for stars to form.”

By assuming that stars form over millions of years, the critic has taken for granted that they were not supernaturally created. He has tacitly assumed the Bible is wrong in his attempt to argue that the Bible is wrong; he has begged the question. Another example is:

“We know evolution must have happened, because we are here!”

This argument begs the question, since the way we got here is the very point in question.

Watch for arguments that subtly presume (in an arbitrary way) what the critic is attempting to prove. In particular, evolutionists will often take for granted the assumptions of naturalism, uniformitarianism, strict empiricism (the notion that all truth claims are answered by observation and experimentation), and sometimes evolution itself. But, of course, these are the very claims at issue. When an evolutionist takes these things for granted, he is not giving a good logical reason for his position; he is simply arbitrarily asserting his position.

Help keep these daily articles coming. Support AiG.

Footnotes

  1. The parting of the Red Sea was certainly a miracle—an extraordinary act of God (Exodus 14:21). Yet, God used wind—a force of nature—to accomplish this miracle. Back
  2. It is always necessary to presuppose the preconditions of intelligibility. These include laws of logic and induction. However, the evolutionist assumes these arbitrarily—without rational justification—and has thus begged the question. The Christian can account for logic and induction within his worldview. Back