The question addressed in this book is whether metaphysics is possible. If metaphysics is a science, why are we unable to make progress or reach unanimous agreements as we can with the other sciences? And if it is not a science, on what grounds do its claims to truth rest? At the moment, there is no standard for agreement on metaphysical questions, so there is no objective means for settling disagreements. As a result, all sorts of opinions are tossed about with no means of reaching definite conclusions.
The question of whether metaphysics is possible implies that the validity of metaphysics can be doubted. This implication may upset many readers: we don't like being told that a subject we have studied intensively might be useless. Nevertheless, Kant has become aware that metaphysics needs a sturdier foundation than it currenty has if it is to be taken seriously. He is confident that those who read his work carefully will agree.
Kant came to recognize the importance of finding a sturdy foundation for metaphysics when he read Hume, whom he claims roused him from a "dogmatic slumber." Hume inspired Kant by critiquing our concept of cause and effect, asking how we know that one event acts as a cause for another event. Hume concludes that we do not have a priori knowledge of causation: we cannot know the causal relationship between two events prior to our experience of it by means of reason alone. Instead, Hume suggests that what we call our "knowledge" of cause and effect is simply an expectation that one event will follow another based on habit rather than reason.
Hume's conclusion is fatal to metaphysics. If our "knowledge" of cause and effect is based on custom and habit rather than reason, then all the metaphysical theories that try to explain how our reason leads us to this knowledge are in vain. On further inspection, Kant found that all metaphysics is based on a priori reasoning, drawing connections between concepts without any reference to experience, so all metaphysics is potentially open to Hume's attack.
Kant explains how connections can be drawn a priori and how metaphysics is possible in his monumental Critique of Pure Reason. This book is long and difficult, however, and so he has written the Prolegomena to any Future Metaphysics as a shorter work that will make the ideas found in the Critique more accessible to a wider audience. The Critique of Pure Reason follows what Kant calls a "synthetical" style, deducing conclusions from first principles. The Prolegomena, on the other hand, follows an "analytical" style, breaking the problem down into simple bits and examining them individually.
Metaphysics is the oldest and most respected branch of philosophy. It examines the constitution, nature, and structure of reality, and strives to uncover the underlying causes and foundations that make things the way they are. Physics simply describes the universe, and the laws of physics are only good for predicting what will happen. Metaphysics, by contrast, tries to explain the universe and why things happen the way they do. While physics is based on observation and experience, metaphysics is an a priori form of knowledge based on the unaided exercise of pure reason. Metaphysicians do no experiments: they try to sort everything out in their heads.