It’s Impossible to Justify Predictions from Evidence. Here’s Why.
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.
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. 1 David Deutsch, 1997, The Fabric of Reality, Chapter 3: Problem-solving, pp. 60-61 |
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