This paper shows how classical and quantum logical entropy arise out of the logic of partitions, and then it shows how there is a natural connection between the nxn distinctions and indistinctions of a partition and the nxn entries in a density matrix so that the classical and quantum logical entropy can directly register what happens to the density matrix in a projective measurement. The standard notion of von Neumann entropy does nothing of the kind–so the paper is also an indirect critique of von Neumann entropy as the most natural and ‘informative’ notion of entropy to use in quantum information theory.