You Can't Have it Both Ways

The Paradox

A main theme here is that one cannot point to scientific evidence when it supports one’s theory and then reject science when it does not.

The Problem of Induction Revisited

We learned that science proceeds through induction. Induction is the process of considering what is known about the past and present, and making predictions about the future. Induction requires the assumption that the the laws of nature will continue to be the same in the future as they have in the past. Science, which proceeds via induction, can only be valued if we assume that the laws of nature will continue in the future.

In fact, science will not work unless nature behaves the same way all of the time.

When an advocate for an idea uses science to support the idea, he/she implies that science has value. We will learn below that if science has any value in evaluating testable claims, then we must take the position that it works all of the time. If science is later used to show that the idea is incorrect, the advocate would logically contradict himself if he rejects science.

You can't have it both ways.

Theories are potential explanations that are constructed by making specific observations and then forming general ideas. They allow us to move from the known to the unknown. Theories become scientific when we form hypotheses about them which can be tested through a deductive process.Without induction there is no science. Induction cannot be logically justified. Hume pointed out that we take for granted that the inductive process (and hence science too) works because it has always worked in the past. In other words, we assume that what has worked in the past will work in the future...because thinking this way has worked in the past and should work in the future. We cannot justify induction without using induction. This reasoning is completely circular. The premise and the conclusion are the same. It states absolutely nothing.

If we cannot logically justify induction, then we cannot logically justify science!

We Still Use Induction

We all use inductive reasoning. All of us. We cannot help it. It appears to be a heuristic that our brains use in a way that is so fundamental that it seems absurd at first glance to think otherwise. Hume implied that it is part of our hard-wiring. We seek patterns in nature and use induction to make predictive models based on those patterns. Science takes this process a step further by testing these models deductively to see which ones make the best predictions. When we use science, we use induction.

Although we cannot justify our use of science logically, we still use it. Once we recognize the circularity of it, we also recognize that using science is a choice. We choose to value it. We do not have to choose it. However, as we will see, if we reject science, then we cannot ever use it.

If inductive reasoning is as fundamental to our thinking as it seems, then we all use it. We may all be prone to using empiric evidence to justify a position, at least from time to time.

Basic Logic Revisited

You will recall that a basic argument involves a simple syllogism.

e.g., “If A, then B”


“Therefore B”

In a conditional syllogism, logic will not allow one necessarily to “Deny the Antecedent”:

e.g., “If A, then B”

“Not A"

“Therefore, not B"

This is a fallacy because we cannot deduce that B will not occur if A does not. B may occur anyway. B may occur because of X, Y or Z. B may always occur no matter what the antecedent. We have no way of knowing and cannot justify the statement.

Logic will not allow one necessarily to “Affirm the Consequent” either:

e.g. “If A, then B”


“Therefore A”

This is a fallacy because of the same reasons. There may be more than one prerequisite for B. B may not need any prerequisites. We have no warrant to make such a conclusion.

It’s Not a Fallacy "If and Only If..."

Now, logic will allow us to “deny the antecedent” if the premise is restrictive to A and B:

e.g., “If and only if A, then B”

“Not A”

“Therefore, not B”

By adding the “if and only if”, we restrict the prerequisites for B to only A. In principle, this would be fine, but in the real world, we rarely have access to such a priori knowledge such that all other potential antecedents can be eliminated.

Similarly, we can “affirm the consequent” if we include “if and only if” in the first premise:

eg. “If and only if A, then B”


“Therefore, A”

Again, this works in principle, but we rarely (if ever) have complete a priori knowledge that B can only occur as a result of A.

We learned above that induction depends on nature behaving in the same way all of the time. Science depends on induction and therefore also depends on nature behaving in the same way all of the time.

The Problems of Science

The principle of science deals with building knowledge about testable concepts.

Remember, we generally accept a concept to be scientific if it is potentially falsifiable. For more on this, please review the What is Science page. When we say that science has value, we are speaking with respect to claims that are testable, that are empiric.

The process of science as a human endeavor tends to be sloppy and often comes up with inconsistent results. However, the problems of science as an activity stem from human biases and fallibilities. Good science reduces these biases and errors as much as possible.

The principle of science should work all of the time if nature is consistent in its behavior all of the time.

Consider the following. One may make an empirical claim, that is then tested by others scientifically. If the test shows that the claim is false, the person making the claim may try to defend it by attacking the science. The claimant may legitimately accuse the scientists of faulty science with the following options:

1. The methodology used was inadequate. This is often the case when the study does not have proper controls, blinding or sample size.

2. There was an error in calculation. Conclusions based on faulty calculations are obviously faulty as well.

3. The conclusion did not follow from the data. If the conclusion is not logically warranted, it is not valid.

4. The authors failed to consider all of the available data. The conclusion is based on the results of select studies, but the results of other studies were ignored. This is confirmation bias.

If none of the above 4 are correct, then the claimant may cling to the conclusion by appealing to the 5th option below:

5. The principle of science does not work all of the time. This implies that theories built from induction and hypothesis testing based on deduction cannot be trusted. Ever. If the process of science does not work at least some of the time, how could we ever trust it?

The first 4 problems are real-world, human problems. Because of these problems, the process of science is admittedly not perfect. However, that is our fault, not science's. The solutions to these problems lie in proper understanding of biases, a mastery of scientific methodology, peer review, knowledge of the philosophy of science, intellectual integrity, and simply being careful.

It has often been said that science is self-correcting. Over time, these human problems get worked out. The principle of science is fine. If it can be shown that these human problems have been properly avoided, then one cannot attack science on these grounds.

Such problems have nothing to do with the validity of the principle of science.

The last problem stems from the problem of induction as discussed above. We have no access to knowing that nature is always consistent with itself. We assume this to be true and therefore assume that inductive reasoning is valid. Science is simply a method of using inductive reasoning and attempting to eliminate errors through deductive testing. Science therefore depends on induction. There is no logical justification for this assumption as it begs the question. However, the assumption that nature is not always internally consistent is equally unjustified. One may choose to reject induction - and hence science - but by doing so, one admits that nothing can be known. This extreme form of skepticism prohibits one from making any empiric claims. The process of science (and modern skepticism) makes the fundamental assumption that induction is valid. Everything is built from that internally consistent worldview. We must reject number 3 if we use induction to justify any position.


** We take it for granted that the principle of science works all of the time, provided that nature behaves in the same way all of the time. This is why we value it. In fact, this is why science could have some (or any) value.

The Logic of Science by Affirming the Consequent

Now, let’s put this into a syllogism. (Note - It is the principle of science that we are referring to; it is the principle of using induction to come up with ideas, and deduction to test the ideas..)

We can let “A” = “The Principle of science works all of the time”, and

we can let “B” = “Science has some value.”

We then have...

“If, and only if, the principle of science works all of the time, then science has some value.”

“The principle of science works all of the time.”

“Therefore, science has some value.”

(Again, we have no direct access to knowledge of science working all of the time. That is the very core of the problem of induction. We choose it anyway and work from there.)

Now, with this syllogism, we can deny the antecedent and still be logically valid. The position of the extreme skeptic may be:

“If, and only if, the principle of science works all of the time, then science has some value”.

“The principle of science does not work all of the time.”

“Therefore, science does not have any value.”

We can affirm the consequent. The position of the modern skeptic may be formulated as:

“If, and only if, the principle of science works all of the time, then science has some value.”

“Science has some value.”

“Therefore, the principle of science works all of the time.”

We choose to believe in induction. Therefore, we choose to believe in the principle of science. We choose the "if, and only if" part of the argument. To do otherwise renders induction and science useless.

The Problem of Pseudoscience

Pseudoscientists often claim that science is (at best) just one way of knowing. When it comes to empirical claims (like the accuracy of astrology), the pseudoscientist is forced to explain away disconfirming evidence by attacking science itself. An example is the astrologer Rob Hand, who is quoted on the Skeptico blog when he defended the practice's lack of scientific thinking:

"The shortest answer that I can give is that rules were probably not derived in a manner that we would regard as scientific. The problem is that it is not clear to many of us that the scientific method is the only source of truth."

"Astrology represents a very different way of looking at the world and reality from what you are probably used to. Part of the process is getting familiar with that way of looking at things. If Science is for you the only possible way of getting at any sort of truth, then it is unlikely that you will find anything in this that will satisfy you. If I tell you that astrological ideas are closer to revelation than to scientific theory (closer does not mean "the same as"), then you will probably dismiss the idea that there could be any kind of truth in it."

Remember, we assume ideally that the principle of science works all of the time. As scientists utilize better and better methodology, the closer their practice of science will get to working all of the time. We must strive toward this ideal. When good scientific methodology is employed, we are simply not in a position to deny the conclusions out of hand.

Now we come to a major point of this site.

Proponents of pseudoscience present evidence for their claims whenever possible. They use testimonials and other anecdotal evidence as if such things were legitimate data. They use inductive reasoning. Flawed, but inductive. They therefore imply that they place at least some value on science.

Proponents of pseudoscience often are faced with disconfirming evidence by good science. When out of options, they may state that science is not equipped to test their claims. They often imply that science does not work all of the time.

In effect, the pseudoscience proponent often denies the antecedent to the above syllogism:

“The principle of science does not work all of the time.”

This may be legitimate, as they can claim the problem of induction. However, they must also deal with the conclusion that science does not have some value.

By claiming that science (the principle of science) does not work all of the time, the pseudoscience proponent denies himself the right to pay lip-service to science in order to support his ideas.

Pseudoscience promotes itself with the window dressings of science. It claims evidence to be scientific. Promoters of pseudoscience demonstrate that they feel that science has at least some value. Yet, by denying disconfirming evidence and implying that the principle of science doesn’t work all of the time, they contradict themselves.

In a debate setting, playing the "problem of induction" card to refute the findings of science seems to be an act of last resort. At first glance, it seems like a way to defuse an opponent's disconfirming evidence. While it may do that, it also defuses one's own argument as well. Philosopher Stephen Law calls this tactic "going nuclear". It destroys all arguments on the playing field. It leaves nothing to discuss.

Since even proponents of pseudoscience rely on inductive principles to conduct their everyday lives (as we all seem to do), the argument is simply a disingenuous defense mechanism. It is intellectually dishonest.


** We cannot simultaneously value science when it supports an idea, and then deny that it has value when it does not.

You cannot have it both ways.

John Byrne, M.D.