-The same general principles of reasoning apply to psychological theories, even though psychology is a much more difficult subject matter and our available theories are correspondingly much poorer and vaguer. There's no way to make precise quantitative predictions about a human's behavior the way we can about the movements of the planets, but we still know _some_ things about humans, which get expressed as high-level generalities that nevertheless admit many exceptions: if you don't have the complicated true theory that would account for everything, then simple theories plus noise are better than _pretending not to have a theory_. As you learn more, you can try to refine more complicated theories that explain some of the anomalies that looked like "noise" to the simpler theory.
+The same general principles of reasoning apply to psychological theories, even though psychology is a much more difficult subject matter and our available theories are correspondingly much poorer and vaguer. There's no way to make precise quantitative predictions about a human's behavior the way we can about the movements of the planets, but we still know _some_ things about humans, which get expressed as high-level generalities that nevertheless admit many exceptions: if you don't have the complicated true theory that would account for everything, then simple theories plus noise are better than _pretending not to have a theory_. As you learn more, you can try to pin down a more complicated theory that explains some of the anomalies that looked like "noise" to the simpler theory.
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+What does this look like for psychological theories? In the crudest form, when we notice a pattern of traits that go together, we give it a name. Sometimes people go through cycles of elevated arousal and hyperactivity, punctuated by pits of depression. After seeing the same distinctive patterns in many such cases, doctors decided to reify it as a diagnosis, ["bipolar disorder"](https://en.wikipedia.org/wiki/Bipolar_disorder).
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+If we notice further patterns _within_ the group of cases that make up a category, we can spit it up into sub-categories: a diagnosis of bipolar I requires a full-blown manic episode, but hypomania and a major depressive episode qualify one for bipolar II.
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+Are bipolar I and bipolar II "really" different conditions?
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