Summary
Kant published the Critique of Pure Reason in
1781. It is very long and almost unreadable due to its dry prose
and complex terminology. Kant tried to ease his readers’ confusion
by publishing the Prolegomena to Any Future Metaphysics two
years later. While it is hardly a page-turner, the Prolegomena is
much briefer than the Critique and much more accessible
in style, making it a valuable entry point to Kant’s metaphysics
and epistemology.
Kant’s primary aim is to determine the limits and scope
of pure reason. That is, he wants to know what reason alone can
determine without the help of the senses or any other faculties.
Metaphysicians make grand claims about the nature of reality based
on pure reason alone, but these claims often conflict with one another.
Furthermore, Kant is prompted by Hume’s skepticism to doubt the
very possibility of metaphysics.
Kant draws two important distinctions: between a priori
and a posteriori knowledge and between analytic and synthetic judgments.
A posteriori knowledge is the particular knowledge we gain from
experience, and a priori knowledge is the necessary and universal
knowledge we have independent of experience, such as our knowledge
of mathematics. In an analytic judgment, the concept in the predicate
is contained in the concept in the subject, as, for instance, in
the judgment, “a bachelor is an unmarried man.” (In this context, predicate refers
to whatever is being said about the subject of the sentence—for
instance, “is an unmarried man.”) In a synthetic judgment, the predicate
concept contains information not contained in the subject concept,
and so a synthetic judgment is informative rather than just definitional.
Typically, we associate a posteriori knowledge with synthetic judgments
and a priori knowledge with analytic judgments. For instance, the
judgment “all swans are white” is synthetic because whiteness is
not a part of the concept of “swan” (a black swan would still be
a swan even though it isn’t white), but it is also a posteriori
because we can only find out if all swans are white from experience.
Kant argues that mathematics and the principles of science
contain synthetic a priori knowledge. For example, “7 + 5 = 12”
is a priori because it is a necessary and universal truth we know independent
of experience, and it is synthetic because the concept of “12” is
not contained in the concept of “7 + 5.” Kant argues that the same
is true for scientific principles such as, “for every action there is
an equal an opposite reaction”: because it is universally applicable,
it must be a priori knowledge, since a posteriori knowledge only tells
us about particular experiences.
The fact that we are capable of synthetic a priori knowledge
suggests that pure reason is capable of knowing important truths. However,
Kant does not follow rationalist metaphysics in asserting that pure
reason has the power to grasp the mysteries of the universe. Instead,
he suggests that much of what we consider to be reality is shaped
by the perceiving mind. The mind, according to Kant, does not passively
receive information provided by the senses. Rather, it actively
shapes and makes sense of that information. If all the events in
our experience take place in time, that is because our mind arranges
sensory experience in a temporal progression, and if we perceive
that some events cause other events, that is because our mind makes
sense of events in terms of cause and effect. Kant’s argument has
a certain parallel to the fact that a person wearing blue-tinted
sunglasses sees everything in a bluish light: according to Kant, the
mind wears unremovable time-tinted and causation-tinted sunglasses,
so that all our experience necessarily takes place in time and obeys
the laws of causation.
Time and space, Kant argues, are pure intuitions of our
faculty of sensibility, and concepts of physics such as causation
and inertia are pure intuitions of our faculty of understanding.
Sensory experience only makes sense because our faculty of sensibility
processes it, organizing it according to our intuitions of time
and space. These intuitions are the source of mathematics: our number
sense comes from our intuition of successive moments in time, and
geometry comes from our intuition of space. Events that take place
in space and time would still be a meaningless jumble if it were
not for our faculty of understanding, which organizes experience
according to the concepts, like causation, which form the principles
of natural science.
