(For other posts in this series, see here.)

In the previous post in this series, I wrote about the fact that however much data may support a theory, we are not in a position to unequivocally state that we have proven the theory to be true. But what if the prediction disagrees with the data? Surely then we can say something definite, that the theory is false?

The philosopher of science Karl Popper, who was deeply interested in the question of how to distinguish science from non-science, used this idea to develop his notion of falsifiability. He suggested that what makes a theory scientific was that it should make predictions that can be tested, saying that “the criterion of the scientific status of a theory is its falsifiability, or refutability, or testability.” (Conjectures and Refutations: The Growth of Scientific Knowledge, 1963, p. 48)

Popper’s motivation for doing this was his opposition to the claims of the supporters of Marxism, Freudian psychoanalysis, and Jungian psychology that their respective theories were scientific. He said that those theories seemed to be so flexible that almost anything that happened could be claimed to be in support of the theory. While supporters of these theories used these alleged successes as demonstrating the strength of their theories, Popper argued the converse: that a theory that could never be proven wrong was not scientific. A scientific theory was one that made risky predictions that laid bare the possibility that a negative result would require the discarding of the theory. A theory whose predictions could never be contradicted by any conceivable data was not a scientific theory.

Popper also said that humans were born with a innate tendency to make conjectures, to construct a universal theory based on whatever data was at hand, and that we held on to that theory until it was refuted (or falsified) by new data, whereupon we replaced it with a new universal theory. This process of conjectures and refutations went on all the time and was how science functioned. He claimed that this model also solved the problem of induction, why we expected that things that had always happened in the past would continue to happen in the future, when logically there was no reason to infer that.

Although Popper’s main goal was to solve what was known as the demarcation problem, i.e., the ability to distinguish science from non-science, his idea of falsifiability seemed to also advance us along the goal of distinguishing truth from falsehood, because if a prediction disagrees with the data, then we can conclude that the theory is false. This feature seems to give us some hope that we can arrive at a true theory by a back door mechanism. If we can enumerate all the possible alternatives to a theory and prove that all but one are false, then the one remaining theory must be true. To quote Sherlock Holmes in The Sign of Four, “When you have eliminated the impossible, whatever remains, however improbable, must be the truth.”

But that option also proves to be illusory, for a purely practical reason. In science, one can never be sure that one has exhausted all the alternatives. There is no limit to the number of theories that can be postulated to explain any given set of phenomena and so showing one or any number of them to be false does not prove that any of the remaining alternatives are true.

This is the fatal flaw of the arguments of almost all religious believers, especially the creationists and the intelligent designers. Their strategy is to argue that there are only two possible explanations for some phenomenon, an intervention by god or an explanation based on naturalistic science. For example, in explaining the diversity of life, the competing theories are said to be evolution by natural selection or a designer/god. They would then seek some phenomenon that had not been convincingly explained by the scientific theory that encompasses it, declare that the scientific theory had thus been falsified, and triumphantly conclude that the phenomenon must be the work of a god. But that is a false dichotomy. Even in the highly unlikely event that some day in the future the theory of evolution by natural selection experiences a serious enough crisis that scientists suspect it to be false, this would not imply that ‘god did it’ would be accepted as the true explanation. There will be no shortage of other scientific theories competing to replace the theory of evolution, all of them having at least some supporting evidence.

This kind of flawed argument is what religious believers advance even now, with the current candidates for god’s intervention being the origin of life, the origin of the universe, the mind, consciousness, intelligence, morality, etc. They have no choice but to pursue this fundamentally flawed strategy because they have no positive evidence for god.

Next: Are theories falsifiable?