This is Philosophy of Science. Franz-Peter Griesmaier

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predicts Observation

       Observation is made

      The first thing to notice here is that confirmation is different from proof. If a theory entails an observation and the observation is made, it doesn’t logically follow that the theory is true. To think otherwise would be to commit a logical fallacy. To see this, consider: “If it rains, the streets are wet. The streets are wet. Thus, it is raining.” But the conclusion doesn’t follow from the premises, because someone could have hosed down the streets, and that’s why they are wet. However, if the prediction is borne out, we can say that the theory has been confirmed to a certain degree. This in turn means that we have some reason for believing the theory.

      Here’s another example following the same pattern. Suppose I want to confirm the hypothesis that all ravens are black. How would I do this? Following Hempel’s advice, I infer a prediction from my hypothesis and then determine whether it is true or not. What follows from the claim that all ravens are black? One thing that comes to mind immediately is that the next raven I see will be black. Thus,

      Raven Theory predicts “Next raven is black”

       Observation “Next raven is black” is made

       Raven Theory is (better) confirmed

      What’s important about this second example is that the theory talks about all ravens – past, present, and future, and everywhere they exist. Suppose I have seen 200 ravens, and all of them have been black. Thus, I inductively infer my Black Raven theory. Now I want to confirm it by using Hempel’s procedure, as we have just done. But, seriously, how much of a confirmation can that one black raven provide? Remember, my theory talks about all ravens.

      3.2.2 Falsification to the Rescue

      Raven Theory predicts “Next raven is black”

       Observation “Next raven is black” is not made

       Raven Theory is falsified

      Popper made falsification the hallmark of his approach. Scientists, on his view, conjecture all sorts of theories and hypotheses. Whether any of them are worth keeping depends on two factors: How much do they tell us about reality, and whether or not they have been falsified. If we find a theory that has a lot of empirical content, and that we have not been able to falsify despite subjecting it to rigorous testing, we consider it as worth keeping. Popper’s technical term for “not falsified after rigorous testing” is “corroborated.” Thus, a theory is corroborated to the degree to which we have tried to falsify it and failed to do so. Highly corroborated theories are those worth keeping.

      Why is having a lot of empirical content important, and how do we measure the content? First, theories with little to no content are not easily falsifiable. Suppose you restrict your raven theory in the following way: “All the ravens in my raven coop are black.” Unless you have overlooked a raven in your coop, nothing can falsify this theory. It is so specific that there are not many falsifiers. As such, it is not a very interesting theory. So, let’s be a little more daring: “All ravens in my yard are black.” Ravens come and go, and so it might turn out that one morning, you get up and discover that a maroon raven is perched on a tree limb in your yard. However, even this daring theory is still a far cry from the original theory we considered, according to which all ravens are black. Clearly, it has a lot more content – talks about a lot more things – than the theory about the ravens in the coop. Since it talks about more things, there are more things that can falsify it. Thus, we can now answer, at least in principle, the question how we should measure the content of a theory. To a first approximation, the content simply is the set of things that could falsify it, were you to observe them.

      3.2.3 Ravens and White Chalk

      The falsification approach has a second advantage: It solves a notorious problem that arises for Hempel’s model of confirmation, a problem of which Hempel himself was aware. The problem itself is a bit on the abstract side and will strike any working scientist as a “typical philosophical problem,” but it is worth considering at this juncture because it is both clever and troubling. Imagine that an ornithologist had spent his career studying ravens around the world. He’s seen a couple thousand of the birds, and every one has been black. So, he’s working on a paper proposing that all ravens are black, but he’s wondering what evidence he can use in addition to his observations of the birds. While lecturing in class, he looks at the chalk in his hand, stops speaking, and a big smile comes to his face. The ornithologist realizes that the white chalk supports his thesis. But how could this be?

      Let us look at the paradox in explicit argument form:

      1 Positive instances confirm a universal generalization, such as the generalization “All ravens are black.”

      2 Logically equivalent theories are confirmed by the same evidence.

      3 “All ravens are black” is logically equivalent to “All nonblack things are nonravens.”

      4 A piece of white chalk is a positive instance of the generalization “All nonblack things are nonravens.”

      5 Thus,

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