Groundwork of the Metaphysics of Morals (German: Grundlegung zur Metaphysik der Sitten; 1785; also known as the Foundations of the Metaphysics of Morals, Grounding of the Metaphysics of Morals and the Grounding for the Metaphysics of Morals) is the first of Immanuel Kant's mature works on moral philosophy and remains one of the most influential in the field. Kant conceives his investigation as a work of foundational ethics-one that clears the ground for future research by explaining the core concepts and principles of moral theory and showing that they are normative for rational agents. Kant aspires to nothing less than this: to lay bare the fundamental principle of morality and show that it applies to us. In the text, Kant provides a groundbreaking argument that the rightness of an action is determined by the character of the principle that a person chooses to act upon. Kant thus stands in stark contrast to the moral sense theories and teleological moral theories that dominated moral philosophy at the time he was writing. Central to the work is the role of what Kant refers to as the categorical imperative, the concept that one must act only according to that precept which he or she would will to become a universal law.
The Groundwork is broken into a preface, followed by three sections (1: Transition from the common rational knowledge of morality to the philosophical; 2: Transition from popular moral philosophy to the metaphysic of morals; 3: Final step from the metaphysic of morals to the critique of the pure practical reason). Kant's argument works from common reason up to the supreme unconditional law, in order to identify its existence. He then works backwards from there to prove the relevance and weight of the moral law. The third and final section of the book is famously obscure, and it is partly because of this that Kant later, in 1788, decided to publish the Critique of Practical Reason.
Summary of The Fundamental Principles of the Metaphysic of Morals (also known as the Groundwork of Metaphysic of Morals) by German Philosopher, Immanuel Kant adapted from Wikipedia.