In recent times there has been some convergence of apparently disparate disciplines like engineering on one hand and social sciences on the other. A portable philosophy of systems analysis and design has evolved through progressive development from loop and node equations to state-space equations, from electrical networks to other physical systems, and from technological systems to transportation and economic systems. This paper addresses itself to the philosophy underlying the modelling side of system theory techniques that are sufficiently powerful to have important application in the planning, operation and control of complex large-scale systems required by modern society.