Ancient Greek philosophy was divided into three fields: logic, physics (natural philosophy), and ethics (moral philosophy). This division makes sense: Logic is the study of pure thought, independent of any objects. Physics is the study of how things happen in the world of material objects. Ethics is the study of how things ought to happen in the world of human beings.
Philosophy may also be divided on the basis of whether it is "pure" or "empirical." Pure philosophy deals only with a priori concepts; concepts that occur to us independent of any experience or perception. By contrast, empirical philosophy deals with the objects we experience in the world around us. Logic is pure philosophy as it relates to the formal procedures of thinking. "Metaphysics" is pure philosophy as it applies to our efforts to understand the world. Physics and ethics have both empirical and metaphysical branches.
Our task in this book is to develop a "pure" moral philosophy, a "metaphysics of morals" that relies on the a priori concepts of reason, not on empirical observations. That such a philosophy should be possible is apparent from the fact that we presume that moral obligations are binding not just for particular people in particular circumstances, but rather for all rational beings in all places at all times.
In their daily lives, people must apply moral laws to many different situations and circumstances. Developing a clear understanding of moral principles can help people to keep track of their moral obligations. A clear understanding of morals can also help people to ensure that their motivations are pure. Actions are not truly moral if they only appear to conform to moral law but lack a moral motivation.
The goal of the Grounding for the Metaphysics of Morals is to establish the "supreme principle of morality." Kant intends to follow this work with a more thorough treatment of moral philosophy. The Grounding is meant to be more accessible than this later work.
The distinction that Kant draws in the Preface between "pure" and "empirical" concepts is of critical importance to his philosophy. "Pure" or "a priori" concepts are ideas that occur to us when we think about things in our minds, "prior" to and independent of any experience of how things happen in the world. "Empirical" or "a posteriori" concepts are ideas that we derive from our experience of the world.