 I am grateful to the following for helpful comments and conversations regarding previous versions of this entry: Laura Callahan, Henk de Regt, Catherine Elgin, John Greco, Crina Gschwandtner, Michael Hannon, Xingming Hu, Tania Lombrozo, Federica Malfatti, Kevin McCain, Keith McPartland, Soazig le Bihan, Angela Potochnik, Constantine Sandis, Armin Schulz, Whitney Schwab, Bhavya Sharma, Brad Skow, Michael Strevens, Karsten Stueber, Emily Sullivan, Daryl Tress, Rene van Woudenberg, Daniel Wilkenfeld, and Bryan Williams. Special thanks to Kareem Khalifa for careful discussion of the final version.