A unified foundation for the linear theory of policy in both its static and dynamic guises has been the overriding objective assailed throughout this book. It is now appropriate, if chastening, to review briefly its salient features, to identify specific contributions intended to inhere in those foundations, and to gloss quickly over its known limitations.

Three fundamental issues fall for analysis in any policy context – existence, uniqueness and design. When the juxtaposition of a linear policy model with a policy objective generates a linear policy mapping from instrument space into target space, the theory of linear mappings affords a substantial resolution of this trio. To be singled out as cornerstones of the book's deployment of that analytical framework are the following results: Tinbergen's Theorem as generalised (Theorem 2.23); the existence/uniqueness duality; the fixed/flexible correspondence; the linear system representation; the Cayley–Hamilton theorem; and the structure algorithm.

Of these, the first three find employment in the analysis of both static and dynamic policy problems, whilst the second three are specific to the analysis of dynamic problems. Tinbergen's theorem, as generalised, provides a definitive statement on policy existence, uniqueness and design and has been appealed to constantly whenever one of these issues has appeared: it is unquestionably the fundamental tool of the linear theory of policy. The existence/uniqueness duality serves two functions: as an economising device for adducing uniqueness criteria from existence criteria (or conversely) and as a unifying device in proclaiming the duality of existence and uniqueness analyses.