Throughout this book, we have tried to keep technical and mathematical tools of use in the study of nonequilibrium phenomena together with the intellectual reflections that accompanied their development. We deem important to keep in mind such foundational aspects of statistical mechanics and nonequilibrium physics, as further and even unpredictable progress is assured. Moving in uncharted territory using the same techniques that have proved successful in the past is, in fact, as inevitable as fraught with danger.