have been educating me, from di¤erent points of view, on many fascinating visions of the thorny concept of complexity. More than a decade and a half of constant, stimulating, discussions with John McCall on Kolmogorovalgorithmic -complexity and theories of probability have been a source of great pleasure and consistent intellectual adventures. Not being particularly competent in the actual programming of theoretically structured computing models, I have had to rely on Stefano Zambelli for instruction and advice on the felicitous link between theory and application that is the hall mark of computational complexity theory. More recently, I have had the pleasure and privilege of being able to pick Chico Doria's fertile brain for exotic ideas about the P =?N P question. Years ago, now bordering on almost two decades, the decision by Shu-Heng Chen to write his UCLA dissertation on the application of algorithmic and stochastic complexity theories, with me as his main thesis advisor, forced me to try to be at least one-step ahead of his incredible learning abilities. Barkley Rosser's stimulating visit to Trento last year provided much inspiration on many topics related to a broader perspective on complexity. I wish I could blame them, and not have to plead guilty, for the infelicities and errors that remain! 1 Abstract A computable economist's view of the world of computational complexity theory is described. This means the model of computation underpinning theories of computational complexity plays a central role. The emergence of computational complexity theories from diverse traditions is emphasised. The uni…ca-tions that emerged in the modern era was codi…ed by means of the notions of e¢ ciency of computations, non-deterministic computations, completeness, reducibility and veri…ability -all three of the latter concepts had their origins on what may be called 'Post's Program of Research for Higher Recursion Theory'. Approximations, computations and constructions are also emphasised. The recent real model of computation as a basis for studying computational complexity in the domain of the reals is also presented and discussed, albeit critically. A brief sceptical section on algorithmic complexity theory is included in an appendix.
21 Prologue "There are many levels of complexity in problems, and corresponding boundaries between them. Turing computability is an outer boundary, ... any theory that requires more power than that surely is irrelevant to any useful de…nition of human rationality. A slightly stricter boundary is posed by computational complexity, especially in its common "worst case" form. We cannot expect people (and/or computers) to …nd exact solutions for large problems in computationally complex domains. This still leaves us far beyond what people and computers actually CAN do. The next boundary... is computational complexity for the "average case" .... .. That begins to bring us closer to the realities of real-world and realtime computation. Finally, we get to the empirical boundary, measured by labora...