Discussions of unified science frequently suppose that the various scientific theories should be combined into one unified theory, and it is usually supposed that this should be done by successive reductions of the various theories to some fundamental theory. Yet, there has been little systematic study of the characteristics of unified theories, and little foundational support for the use of reductions as a unifying procedure.
In this paper I: (a) briefly review some of my previous work on microreductions, (b) state some conditions which are necessary in order for a theory to be unified, (c) argue that when certain identities exist between the elements in the domains of two theories, then the only satisfactory way to combine these two theories into one unified theory is by a microreduction, and (d) indicate briefly some further applications and consequences of this work.
