Leveraging the Boolean intent of digital circuits has enabled a wide set of CAD tools that helped increase the productivity of digital designers. To increase analog designers' productivity requires a similar encapsulation of designer's intent for analog circuits. We argue that linear system models serve this role for almost all analog circuits, while the variables of these models may be in some transformed domains, rather than being the direct voltage/current waveforms of the circuits. We show how using these models enable new ways to design, optimize, and validate mixed-signal circuits. Even systems that reach steady states only in a stochastic sense can be analyzed as linear systems. Then a remaining issue is to ensure that the non-linear circuit reaches the intended ldquolinearrdquo operating point during start-up, which can be addressed by global convergence analysis.