Abstract Typicality is routinely invoked in everyday contexts: bobcats are typically short-tailed; people are typically less than seven feet tall. Typicality is invoked in scientific contexts as well: typical gases expand; typical quantum systems exhibit probabilistic behaviour. And typicality facts like these support many explanations, both quotidian and scientific. But what is it for something to be typical? And how do typicality facts explain? In this paper, I propose a general theory of typicality. I analyse the notion of a typical property. I provide a formalism for typicality explanations, and I give an account of why typicality explanations are explanatory. Along the way, I show how typicality facts explain a variety of phenomena, from everyday phenomena to the statistical mechanical behaviour of gases. Finally, I argue that typicality is not the same thing as probability. 1Introduction2An Analysis of Typicality3Typicality Explanation3.1Examples of typicality explanations3.2A formalism for typicality explanation3.3An account of explanatory typicality facts4Differences between Probability and Typicality4.1Explanatory differences4.2Formal differences4.3Metaphysical differences5Conclusion

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I show that standard grounding conditions contradict standard conditions for the identities of propositions.

I show that centered propositions-also called de se propositions, and usually modeled as sets of centered worlds-pose a serious problem for various versions of Lewis's Principal Principle. The problem, put roughly, is that in scenarios like Elga's 'Sleeping Beauty' case, those principles imply that rational agents ought to have obviously irrational credences. To solve the problem, I propose a centered version of the Principal Principle. My version allows centered propositions to be objectively chancy.

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