For seventy years, programming languages have inherited two silent assumptions from the von Neumann tradition. The first is that a program's semantics end at the edge of its process, so that frontend and backend, managed and native, script and service are separate programs, joined by glue code, marshaling layers, and toolchains the programmer must operate by hand. The second is that computation stands still while data is delivered to it. This book presents Jac, a production language that discards both assumptions, and develops the theory, design, and implementation of the two language classes it defines: the synechic languages, which present a single continuous semantic medium across the substrates convention treats as separate, and the topokinetic languages, in which the locus of computation moves, as a first-class construct, through a topology of data. The two classes are independent, and Jac is the first language that is a member of both. The book draws on the language's evolution across a decade of research and production deployment. Jac is its existence proof: one production system in which both inherited assumptions are discarded at once. It is written not just for programming-language researchers and graduate students, but also systems builders, and passionate developers of every kind who want a rigorous account of what programming looks like when the discontinuities are removed.