  In Picard-Vessiot theory, the Galois theory for linear differential
equations, the Picard-Vessiot ring plays an important role, since it is the
Picard-Vessiot ring which is a torsor (principal homogeneous space) for the
Galois group (scheme). Like fields are simple rings having only (0) and (1) as
ideals, the Picard-Vessiot ring is a differentially simple ring, i.e. a
differential ring having only (0) and (1) as differential ideals. Having in
mind that the classical Galois theory is a theory of extensions of fields, i.e.
of simple rings, it is quite natural to ask whether one can also set up a
Picard-Vessiot theory where the base is not a differential field, but more
general a differentially simple ring. It is the aim of this article to give a
positive answer to this question, i.e. to set up such a differential Galois
theory.
