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I often find that my understanding of the world is strongly informed by a few key concepts. For example, I've repeatedly found the concept of opportunity cost to be a useful frame. My previous post on privileging the question is in some sense about the opportunity cost of paying attention to certain kinds of questions (namely that you don't get to use that attention on other kinds of questions). Efficient charity can also be thought of in terms of the opportunity cost of donating inefficiently to charity. I've also found the concept of incentive structure very useful for thinking about the behavior of groups of people in aggregate (see perverse incentive).
I'd like people to use this thread to post examples of concepts they've found particularly useful for understanding the world. I'm personally more interested in concepts that don't come from the Sequences, but comments describing a concept from the Sequences and explaining why you've found it useful may help people new to the Sequences. ("Useful" should be interpreted broadly: a concept specific to a particular field might be useful more generally as a metaphor.)
But they sort of do, right? They use "metaphysics" to refer to a tradition in philosophy started by what Aristotle talked about in the books anthologized after his works on physics. My point is I certainly would not cash out "metaphysics" to "the physics of physics" in the same way that I would cash out "metamathematics" to "the mathematics of mathematics."
If you think ethics is the study of the question "what should we do?" then metaethics is the study of the question "how should we determine what we should do?"
This is merely a curiosity, but I think it does make non-negligible sense to think of metaphysics as the physics of physics. Abstracting from regularities of experience to regularities of regularities of experience. Metaphysics tells us what laws of physics are logically possible in the same way physics tell us what patterns of experience are physically likely.