The paper proposes an empirical methodology for understanding the nature and behavior of Sustainable Development as a vague and multidimensional concept by a case study of participatory and demand determined Rural Drinking water Supply systems in India. It combines for the first time, two of the most influential models – ‘Supervaluationism’ and ‘Degree Theory’- on the measurement of ‘Vagueness’, for timely public intervention in reversing the process of Un-sustainability. Analysis clearly brings out the role of institutional, financial and environmental factors that should be part of Public Policy, for ensuring sustainability of potable water supply