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.
The nature of causation is an important topic in metaphysics. We can see in our day to day life that certain events seem to cause other events: one billiard ball may cause another billiard ball to move, or a fall from a great height may be the cause of a broken leg. The metaphysical question, then, is why and how one event can act as a cause for another. How do we know that a certain event is the cause of a later event, and not just a coincidental precedent? What is the nature of the causal connection between the two?
Hume's answer, in short, is that there is no discernable difference between two events that are related causally and two events that are just coincidentally conjoined. He argues that we say two events are causally connected if we see them frequently conjoined. Hume does not believe that we have a rational justification for doing so. We do not and cannot perceive the causal connection itself, and all our talk about cause and effect is based simply on the habit of seeing certain events happen one after another.
Kant notes that Hume's argument is an attack on the very possibility of doing metaphysics. Metaphysics tries to look behind the events themselves and see the fundamental connections and inner workings that tie things together. As a result, metaphysics relies on the assumption that the intellect has the power to see these fundamental connections and inner workings even if the senses do not. Hume's assertion that the intellect has no such power is thus a fatal blow to the very study of metaphysics.
Kant is willing to agree with Hume, but he is not as content as Hume is to simply conclude that metaphysics is misguided. Kant concludes instead that metaphysics is in need of clearer definition and stronger foundation if it is to be taken seriously. He complains that metaphysics is unscientific, that there are no standards for right and wrong, and that anybody's opinion is as good as anybody else's.
Kant's project, then, is to make metaphysics scientific. This means turning metaphysics into a systematic body of knowledge built on first principles. Newtonian physics, for instance, begins with Newton's three laws, which are based on careful observation and experience. Further physical principles are then deduced from these three laws. A new proposition can then be judged to be true or false quite easily based on whether or not it accords with the laws and principles that are already in place. Kant hopes to do the same for metaphysics so that disagreements and criticisms regarding metaphysical problems can be settled objectively, once and for all.
This project is part of what is called Kant's "critical" period. In his early career, he followed in the footsteps of rationalist metaphysicians such as Descartes, Leibniz, and Christian Wolff. The influence of Hume led Kant to write his three great "critiques": the Critique of Pure Reason, the Critique of Practical Reason, and the Critique of Judgment. These works, along with the Prolegomena, are "critiques" because they do not simply try to answer metaphysical questions, but ask instead how we know or how we claim to know the answers to these questions. Kant is primarily interested in knowing, for instance, how we can know that two events are connected causally, rather than what the nature of that causal connection is.