The Ptolemy Project is indeed a study of models of systems. The systems, however, are quite different from the ones I focused on in the past. Those were given to me by nature, but the ones in this book are created by humans. In this book, the purpose of modeling is to improve systems, and there is nothing I could have done to improve the planetary system given to us by nature.

In short, in engineering, as opposed to science, models play a role in the design of the systems being modeled. As with science, models can be improved, but unlike science, so can the systems being modeled.

Even more interestingly, in engineering, unlike science, the choice of models affects the systems being modeled. Two engineers, given the same goals, may come up with radically different designs and realizations of a system simply because they started with radically different models of the system. Moreover, these two engineers may have come up with different models simply because they started with different tools for constructing models.

An engineer who constructs models with pencil and paper will likely come up with very different models than those of an engineer who starts with a software modeling tool. As a consequence, they will likely come up with very different system designs.

This text collects an astonishingly rich variety of modeling tools and techniques for complex systems. Some of these will undoubtedly be improved upon in the future, like my own epicycles, a modeling complexity that was rendered unnecessary by Copernicus. Nevertheless, the goal of this book is to offer to engineers the best modeling techniques available today, with the hope that this will lead to the best system designs achievable today. Tomorrow, we can be sure we will be able to do still better