Two well-known results of E.
C. Posner state that the composition of two nonzero derivations of a prime ring
cannot be a nonzero derivation, and that in a prime ring, if the commutator of each
element and its image under a nonzero derivation is central, then the ring is
commutative. Our purpose is to show how the theory of differential identities can be
used to obtain these results and their generalizations to Lie ideals and to rings with
involution.