In “An ‘Entirely Different Series of Categories,’” I argue that aside from Peirce’s formal categories of Firstness, Secondness, and Thirdness, Peirceans should acknowledge a second set of categories I call the material categories. I also argue that the material categories are irreducible to the formal categories. However, in that article I offer no account of what the material categories are. Moreover, Peirce himself never provides a clear and explicit account of them. The present essay attempts to provide an account of (...) the Peircean material categories. Like Peirce’s “Guess at the Riddle,” which sought to elucidate his formal categories, this is a guess at the other riddle, his material categories, with.. (shrink)