A simple proof of Dieudonné-Manin classification theorem

The Dieudonné-Manin classification theorem on .-modules (.-isocrystals) over a perfect field plays a very important role in p-adic Hodge theory. In this note, in a more general setting we give a new proof of this result, and in the course of the proof, we also give an explicit construction of the Harder-Narasimhan filtration of a .-module.