Imagine a chicken on a farm who sees the farmer bring it food every morning. Every day, the chicken watches the farmer come with food, making it think that the farmer always brings food. But one day, the farmer comes not to feed the chicken but to take it to the slaughterhouse.

This simple story, first told by Bertrand Russell, illustrates the problem of induction: the idea that repeated observations cannot logically justify scientific theories. But as David Deutsch points out in his book The Fabric of Reality¹, this story entirely misses (and accepts) a more fundamental misconception: that extrapolating observations to form new theories is possible in the first place.

You can’t make sense of observations without fitting them into a theory first. In the above example, for the chicken to arrive at its false prediction, it must first have guessed an incorrect explanation of the farmer’s behavior. Maybe the chicken thought the farmer was simply benevolent towards it and wanted to feed it to keep it healthy and happy. Had the chicken guessed the correct explanation for the farmer’s behavior, namely, that the farmer was trying to fatten the chicken up for slaughter, it would have understood the farmer’s actions differently.

The same observation can lead to entirely different predictions depending on the explanation chosen, and this isn’t just a farm issue—it is universal and happens with all observations in any situation. Evidence cannot imply, justify, or support a theory. A particular piece of evidence is consistent with an infinitude of theories, including theories predicting every logically possible outcome of a specific experiment.

This was the problem philosopher Karl Popper was thinking about during the summer of 1919. He grew dissatisfied with three theories in particular: the Marxist theory of history, Freud’s psychoanalysis, and Alfred Adler’s individual psychology, and began to feel dubious about their claims to scientific status.

“I found that those of my friends who were admirers of Marx, Freud, and Adler, were impressed by a number of points common to these theories, and especially by their apparent explanatory power. These theories appeared to be able to explain practically everything that happened within the fields to which they referred. …

The most characteristic element in this situation seemed to me the incessant stream of confirmations, of observations which ‘verified’ the theories in question; and this point was constantly emphasized by their adherents. A Marxist could not open a newspaper without finding on every page confirming evidence for his interpretation of history; not only in the news, but also in its presentation—which revealed the class bias of the paper—and especially of course in what the paper did not say. The Freudian analysts emphasized that their theories were constantly verified by their ‘clinical observations’. As for Adler, I was much impressed by a personal experience. Once, in 1919, I reported to him a case which to me did not seem particularly Adlerian, but which he found no difficulty in analysing in terms of his theory of inferiority feelings, although he had not even seen the child. Slightly shocked, I asked him how he could be so sure. ‘Because of my thousandfold experience’, he replied; whereupon I could not help saying: ‘And with this new case, I suppose, your experience has become thousand-and-one-fold.’

What I had in mind was that his previous observations may not have been much sounder than this new one; that each in its turn had been interpreted in the light of ‘previous experience’, and at the same time counted as additional confirmation. What, I asked myself, did it confirm? No more than that a case could be interpreted in the light of the theory. But this meant very little, I reflected, since every conceivable case could be interpreted in the light of Adler’s theory, or equally of Freud’s. … I could not think of any human behaviour which could not be interpreted in terms of either theory. It was precisely this fact—that they always fitted, that they were always confirmed—which in the eyes of their admirers constituted the strongest argument in favour of these theories. It began to dawn on me that this apparent strength was in fact their weakness.”

— Karl Popper, 1963, Conjectures and Refutations, Chapter 1: Science: Conjectures and Refutations

Popper discovered that for a theory to be scientific, there must be an event that could refute the theory in question. Irrefutability is a bug, not a feature. Remember: Any finite number of observations can be accommodated within an indefinitely large number of different explanations. As Popper would say, all observation is theory-laden.

In science, evidence only helps choose among competing theories by disproving those that don’t match it. Even the most accurate predictions are no substitute for the underlying explanations. An incorrect prediction automatically makes the underlying explanation unsatisfactory, but a correct prediction says nothing at all about the underlying explanation.

The main goal of science is to explain reality, not just observe or predict things, which inevitably require a theory to make sense of them.