1. Falsifiability

2. Friedrich von Langenfeld, a Jesuit priest, wrote the Cautio Criminalis (1631). In it he mocked witch trials: Friedrich von Langenfeld, a Jesuit priest, wrote the Cautio Criminalis (1631). In it he mocked witch trials: On being accused, a woman might run or stay; if she ran, that proved her guilt; if she stayed, the devil had kept her so she could not get away. On being accused, a woman might run or stay; if she ran, that proved her guilt; if she stayed, the devil had kept her so she could not get away. If the witch had led an bad life, she was guilty. If the witch had led an bad life, she was guilty. If she had led a good life, she was guilty, for witches always try to appear virtuous. If she had led a good life, she was guilty, for witches always try to appear virtuous. Once in prison: if she was afraid, this proved her guilt. Once in prison: if she was afraid, this proved her guilt. If she was not afraid, this proved her guilt, for witches have magic powers. If she was not afraid, this proved her guilt, for witches have magic powers.

3. Witchcraft: An unfalsifiable belief Langenfeld acted as confessor to many witches; he was in a position to observe all outcomes. He knew that no matter what the accused witch said or did, it would count against her. Langenfeld acted as confessor to many witches; he was in a position to observe all outcomes. He knew that no matter what the accused witch said or did, it would count against her. The charge of being a witch was unfalsifiable: no evidence could show you were not a witch, that is, no evidence could falsify it. The charge of being a witch was unfalsifiable: no evidence could show you were not a witch, that is, no evidence could falsify it. (This is why scientists predict before experiment.) (This is why scientists predict before experiment.)

4. Falsifiability: Karl Popper Popper (1902-1994) argued that, to be scientific, “it must be possible for an empirical scientific system to be refuted by experience.” Evidence does not prove a hypothesis is true, but can show it is false.

5. Falsifiability is not verification Consider “All ravens are black”. One could never prove (verify) that claim, no matter how many ravens one finds. (cf. Hume) But it takes only one non-black raven to disprove or falsify it. This asymmetry underlies Popper’s argument for the primacy of falsification over verification.

6. Falsification: Modus Tollens If P then Q If P then Q Q is false. Q is false. Therefore, P is false. Therefore, P is false. This is a logically valid argument pattern. This is a logically valid argument pattern.

7. Falsification: Modus Tollens Suppose a theory implies “this sample of metal will expand when heated.” Suppose a theory implies “this sample of metal will expand when heated.” So: “If this theory is true, then this metal will expand under heat.” So: “If this theory is true, then this metal will expand under heat.” The sample is heated but does not expand. The sample is heated but does not expand. Therefore, the theory is false. Therefore, the theory is false. (Popper thinks this shows that science doesn’t rely on induction.)

8. Popper and Demarcation Strict empiricists distinguish science from non- science by saying science must be based on experience. Strict empiricists distinguish science from non- science by saying science must be based on experience. Popper demarcates science from non-science by saying science is falsifiable by experience. Popper demarcates science from non-science by saying science is falsifiable by experience. A statement is falsifiable if it rules out some observable state of affairs or event. It does not allow all possibilities, like witch trials. A statement is falsifiable if it rules out some observable state of affairs or event. It does not allow all possibilities, like witch trials.

9. Falsifiable claims G = Mm/d 2 is falsifiable: the law could have been G = Mm/d 1.3 or any other value. G = Mm/d 2 is falsifiable: the law could have been G = Mm/d 1.3 or any other value. “All metals expand when heated” is falsifiable: the next bit of copper heated might not expand. “All metals expand when heated” is falsifiable: the next bit of copper heated might not expand. “Humans evolved from primates” is falsifiable: we could find human fossils older than any other primates’, or our DNA could crucially differ from primate DNA. “Humans evolved from primates” is falsifiable: we could find human fossils older than any other primates’, or our DNA could crucially differ from primate DNA. “The planets orbit in ellipses” is falsifiable: they could have been circles or squares. “The planets orbit in ellipses” is falsifiable: they could have been circles or squares.

10. Unfalsifiable claims “The gravitational force is some function of some variables.” “The gravitational force is some function of some variables.” “For every house, there is a buyer.” “For every house, there is a buyer.” “Everything happens for a reason.” “Everything happens for a reason.” “Your dreams are actually about sexual desire for your father.” “Your dreams are actually about sexual desire for your father.” “Communism will eventually overthrow capitalism.” “Communism will eventually overthrow capitalism.” “Your love life will take a turn for the better.” “Your love life will take a turn for the better.”

11. Unfalsifiable claims Take the example: ‘Your dream is about sexual desire for your father.’ Take the example: ‘Your dream is about sexual desire for your father.’ If this claim is rejected, the psychoanalyst says the true desire is just being denied. If this claim is rejected, the psychoanalyst says the true desire is just being denied. Or: ‘Your love life will get better.’ Or: ‘Your love life will get better.’ If you have just broken up with someone: “Well, that will be for the better!” If you have just broken up with someone: “Well, that will be for the better!”

12. Popper’s ‘Bullshit Detector’ If someone claims to have a theory about the world, ask that person: under what circumstances would you give it up? If someone claims to have a theory about the world, ask that person: under what circumstances would you give it up? If the answer is “never”, then it’s unfalsifiable bullshit! Like the witch trials, the theory could be twisted to fit any evidence and still claim to be ‘true’. If the answer is “never”, then it’s unfalsifiable bullshit! Like the witch trials, the theory could be twisted to fit any evidence and still claim to be ‘true’.

13. Predictions So the more falsifiable a theory is – the more it rules out – the better it is. E.g.: Einstein’s theory of relativity predicted a precise bending of light around the sun. Einstein’s theory of relativity predicted a precise bending of light around the sun. Evolution predicted an inheritable but changeable basis for traits (i.e., genes). Evolution predicted an inheritable but changeable basis for traits (i.e., genes).

14. Popper’s Scientific Method Popper’s model of science: Make bold explanatory conjectures. Make bold explanatory conjectures. Deduce unexpected (but falsifiable) predictions from them. Deduce unexpected (but falsifiable) predictions from them. Try to falsify your conjectures by testing your predictions. Try to falsify your conjectures by testing your predictions. Reject what is falsified; work with what survives. Reject what is falsified; work with what survives.

15. Popper’s view of science Science, then, is not certain knowledge. It is not Nature’s Laws, nor empirical ‘Facts’. Science, then, is not certain knowledge. It is not Nature’s Laws, nor empirical ‘Facts’. “We must not look upon science as a ‘body of knowledge’, but rather as a system of hypotheses which in principle cannot be justified, but with which we work as long as they stand up to tests, and of which we are never justified in saying that we know they are ‘true’ or ‘more or less certain’ or even ‘probable’.” (Logic of Scientific Discovery) “We must not look upon science as a ‘body of knowledge’, but rather as a system of hypotheses which in principle cannot be justified, but with which we work as long as they stand up to tests, and of which we are never justified in saying that we know they are ‘true’ or ‘more or less certain’ or even ‘probable’.” (Logic of Scientific Discovery)

16. Simple observations?

17. Do you believe it because you see it? Or see it because you believe it? One common-sense view of science (traceable back to Francis Bacon (1561-1626)) is that science is based on facts: on undeniable, publicly verifiable, objective observations. One common-sense view of science (traceable back to Francis Bacon (1561-1626)) is that science is based on facts: on undeniable, publicly verifiable, objective observations. However… However…

25. A problem for ‘facts’ What we see depends on the context in which it appears, or on what we already believe (the background, our prior beliefs, our theories). What we see depends on the context in which it appears, or on what we already believe (the background, our prior beliefs, our theories). Consider the claim ‘the earth does not move.’ 500 years ago, almost everyone would agree that this claim was an obvious ‘fact’ that we could just see was true. To reject it, we need theories such as inertia to revise our ‘observation.’ Consider the claim ‘the earth does not move.’ 500 years ago, almost everyone would agree that this claim was an obvious ‘fact’ that we could just see was true. To reject it, we need theories such as inertia to revise our ‘observation.’

26. Facts depend on theories So we can’t simply appeal to ‘facts’ as the basis for science or common sense. So we can’t simply appeal to ‘facts’ as the basis for science or common sense. What I see partly depends on my beliefs, beliefs assumed in the words I use to express what I see, and what parts I choose to pay attention to. What I see partly depends on my beliefs, beliefs assumed in the words I use to express what I see, and what parts I choose to pay attention to. We have to be on guard against the possibility that supposedly simple observations of ‘fact’ are theory-laden: they wouldn’t be “seen” if not for prior beliefs. We have to be on guard against the possibility that supposedly simple observations of ‘fact’ are theory-laden: they wouldn’t be “seen” if not for prior beliefs.

27. Popper on knowledge For Popper, humans have biological and social tendencies, and we interpret the world through those theories. For Popper, humans have biological and social tendencies, and we interpret the world through those theories. But our theories may be wrong. But our theories may be wrong. We should let our theories die in our stead. (Contrast the frog, which has a built-in theory that it cannot doubt.) We should let our theories die in our stead. (Contrast the frog, which has a built-in theory that it cannot doubt.)

28. Popper on knowledge Our individual certainty or doubt is not the best guide to truth. (Notice how different Popper is from Descartes or Hume.) Our individual certainty or doubt is not the best guide to truth. (Notice how different Popper is from Descartes or Hume.) We can’t ground-clear. We don’t start with ‘facts’ or uninterpreted experiences. We always start from theories. We can’t ground-clear. We don’t start with ‘facts’ or uninterpreted experiences. We always start from theories. We only test one theory at a time, while taking others for granted, at least for the moment. We only test one theory at a time, while taking others for granted, at least for the moment. But no worthwhile theory is immune from criticism. Knowledge is fallible. But no worthwhile theory is immune from criticism. Knowledge is fallible.

29. Popper on knowledge Do you claim to know something? Do you claim to know something? Then make a claim that could be proved wrong, and test it. And be ready to change your mind. Then make a claim that could be proved wrong, and test it. And be ready to change your mind. That’s all knowledge is: accepting a claim based on good reasons. You’re betting on it. But: we could be wrong. The only confidence we can have in beliefs is that they are falsifiable, and that they withstand our best tests. That’s all knowledge is: accepting a claim based on good reasons. You’re betting on it. But: we could be wrong. The only confidence we can have in beliefs is that they are falsifiable, and that they withstand our best tests.

30. Criticisms of Popper Popper does not allow any positive knowledge, only theories that haven’t been falsified. So he is really a skeptic. Popper does not allow any positive knowledge, only theories that haven’t been falsified. So he is really a skeptic. We can’t definitively falsify theories: you can always shift the blame onto another theory. We can’t definitively falsify theories: you can always shift the blame onto another theory.

31. Duhem Impossibility of Conclusive Falsification or Verification: No theory is made in isolation. If the overall hypothesis succeeds or fails, there could still be incorrect claims made in the supporting test assumptions. As physicist Pierre Duhem noted, one can save any hypothesis if one is prepared to sacrifice enough other theories.

32. Shifting the blame onto another theory Our hypothesis: the hare is faster than the tortoise. Our hypothesis: the hare is faster than the tortoise. So we predict: the hare will outrace the tortoise. So we predict: the hare will outrace the tortoise. We then observe: the tortoise wins. We then observe: the tortoise wins. The hypothesis that the hare is faster than the tortoise is NOT thereby falsified because we could say another hypothesis was falsified instead. For example: The hypothesis that the hare is faster than the tortoise is NOT thereby falsified because we could say another hypothesis was falsified instead. For example: The hare did not stop in the middle of the race for a nap. The hare did not stop in the middle of the race for a nap. The hare did not get run over while crossing the road. The hare did not get run over while crossing the road. The hare did not get entangled in a philosophical discussion about the rationality of scientific methods with his friend gopher before crossing the finish line. The hare did not get entangled in a philosophical discussion about the rationality of scientific methods with his friend gopher before crossing the finish line.

33. A real example of shifting the blame A neutron can decay into a proton and electron (beta decay). A neutron can decay into a proton and electron (beta decay). In the 1920s physicists found the combined energies of the proton and electron were less than the original neutron’s. This seems to falsify the principle of conservation of energy. In the 1920s physicists found the combined energies of the proton and electron were less than the original neutron’s. This seems to falsify the principle of conservation of energy. Pauli suggested another particle is emitted: invisible, tiny, and electrically neutral. Fermi called it the “neutrino”. Pauli suggested another particle is emitted: invisible, tiny, and electrically neutral. Fermi called it the “neutrino”. At the time, there was no way to detect neutrinos, so Popper would have said “falsified.” At the time, there was no way to detect neutrinos, so Popper would have said “falsified.” But neutrinos were detected in 1956. Conservation of energy is correct. (Or so it seems!) But neutrinos were detected in 1956. Conservation of energy is correct. (Or so it seems!)

34. Can we ever falsify? If a prediction of a theory is found to be false, and we can’t think of a revised hypothesis that leads to new testable predictions, then Popper says we must conclude that the theory is false. If a prediction of a theory is found to be false, and we can’t think of a revised hypothesis that leads to new testable predictions, then Popper says we must conclude that the theory is false. But it could be that the theory is true and the other hypotheses are responsible for the failed prediction, and those others can’t be tested yet. But it could be that the theory is true and the other hypotheses are responsible for the failed prediction, and those others can’t be tested yet. So can we ever conclusively disprove a theory? And why believe current theories? So can we ever conclusively disprove a theory? And why believe current theories?