THE AGE OF REVOLUTION 169
might say that we learn what space
is by observing things in space; and
we learn what substance is from
our observation that the features
of things change without the
underlying thing itself changing.
For instance, though a tree’s leaves
turn from green to brown, and
eventually fall from the tree, it is
still the same tree.
Space and substance
Kant’s arguments show that, on
the contrary, space is an a priori
intuition. In order to learn about
things outside of me, I need to
know that they are outside of me.
But that shows that I could not
learn about space in this way: how
can I locate something outside of
me without already knowing what
“outside of me” means? Some
knowledge of space has to be
assumed before I can ever study
space empirically. We must be
familiar with space a priori.
This argument has an
extraordinary consequence.
Because space itself is a priori, it
does not belong to things in the
world. But our experience of things
in space is a feature of our
sensibility. A thing-in-itself—Kant’s
term for a thing that is considered
separately from sensibility, and
therefore exterior to our minds—
may have nothing to do with space.
Kant used similar arguments to
prove the same thing of time.
Kant then turns to proving the
existence of a priori concepts, such
as substance. He asks us first to
distinguish between two types of
alteration: variation and change.
Variation concerns the properties
that things have: for instance, a
tree’s leaves may be green or
brown. Change is what the tree
does: the same tree changes its
leaves from green to brown. To
make this distinction is already to
use the notion of substance: the
tree (as substance) changes, but
the leaves (as the properties of
substance) vary. If we do not accept
this distinction, then we cannot
accept the validity of the concept
of substance. We would be saying
that any time there is alteration,
something “pops” into or out of
existence; the tree with green
leaves is annihilated at the same
time that the tree with brown
leaves begins to exist from nothing.
Kant needs to prove that such a
view is impossible. The key to this
is time determination. Time cannot
be directly experienced (it is not a
thing); rather, we experience time
through things that alter or do not
alter, as Kant has already shown. If
we experienced time through the
tree with green leaves and also
experienced time through the tree
with brown leaves without there
being any connection between the
two, then we would be experiencing
two separate real times. Since this
is absurd, Kant believes he has
demonstrated that the concept of
substance is absolutely essential
before we can gain any experience
of the world. And, since it isthrough that experience that we
learn anything empirical, the
concept of substance could not
be empirical: it is rather a priori.The limits of knowledge
A philosophical position that
asserts that some state or activity
of the mind is prior to and more
fundamental than things we
experience is called idealism,
and Kant calls his own position
“transcendental idealism.” He
insists that space, time, and
certain concepts are features of
the world we experience (what
Kant called the phenomenal world)
rather than features of the world
itself considered separately from
experience (what Kant called the
noumenal world).
Kant’s claims about a priori
knowledge have both positive
and negative consequences. The
positive consequence is that the
a priori nature of space, time, and
certain concepts is what makes our
experience of the world possible
and reliable. Space and time make
our experience mathematical in
nature; we can measure it against
known values. A priori concepts
such as substance make it possible
to address questions about nature
such as “Is that a substance?” and
“What properties does it exhibit ❯❯Only from the
human standpoint can
we speak of space.
Immanuel KantOur understanding that entities such
as trees undergo change presupposes an
a priori grasp of the concept “substance”,
according to Kant. Such concepts are
the preconditions of our experience.