In an age of soft law and globalization, it has become increasingly difficult for the law to “lay down the law”. To ensure in this new context that its impact is as fair as optimal, legistic must go beyond mere clear legislative drafting: it has to propose a method for integrating the strength of soft law in the democratic law-making process. This treatise presents the foundations of the sciences of legislation (legistic or legisprudence), historically constituted to make laws more rational and effective, clearer and shorter. It explains how to: create an integrated impact assessment that factors in all the components of sustainable development, among the multiplicity of public and private policy instruments (ex ante material legistic); formulate and communicate clear and coherent texts (formal legistic); retrospectively evaluate laws and public policies through methods which should become more participative (ex post material legistic). This treatise is based on Swiss and European Union law, with a look on comparative law. It includes developments in regulation and regulatory governance, behavioral economics, affective sciences and artificial intelligence. This work is intended not only for lawyers, but also for social scientist, members of political and administrative authorities, and anyone interested in public policy evaluation.