The Sleepwalkers (160 page)

Thus,
for
instance,
he
knew
that
his
inverse
ratio
"law"
(between
a
planet's
speed
and
solar
distance)
was
incorrect.
His
thirty-second
chapter
ends
with
a
short,
almost
off-hand
admission
of
this.
But,
he
argues,
the
deviation
is
so
small,
that
it
can
be
neglected.
Now
this
is
true
for
Earth
with
its
small
eccentricity,
yet
not
at
all
true
for
Mars,
with
its
large
eccentricity.
Yet
even
toward
the
end
of
the
book
(in
chapter
60),
long
after
he
had
found
the
correct
law,
Kepler
speaks
of
the
inverse-ratio
postulate
as
if
it
were
true
not
only
for
earth,
but
also
for
Mars.
He
could
not
deny,
even
to
himself,
that
the
hypothesis
was
incorrect;
he
could
only
forget
it.
Which
he
promptly
did.
Why?
Because,
though
he
knew
that
the
postulate
was
bad
geometry,
it
made
good
physics
to
him,
and
therefore
ought
to
be
true.
The
problem
of
the
planetary
orbits
had
been
hopelessly
bogged
down
in
its
purely
geometrical
frame
of
reference,
and
when
Kepler
realized
that
he
could
not
get
it
unstuck,
he
tore
it
out
of
that
frame
and
removed
it
into
the
field
of
physics.
This
operation
of
removing
a
problem
from
its
traditional
context
and
placing
it
into
a
new
one,
looking
at
it
through
glasses
of
a
different
colour
as
it
were,
has
always
seemed
to
me
of
the
very
essence
of
the
creative
process.
³⁵
It
leads
not
only
to
a
revaluation
of
the
problem
itself,
but
often
to
a
synthesis
of
much
wider
consequences,
brought
about
by
a
fusion
of
the
two
previously
unrelated
frames
of
reference.
In
our
case,
the
orbit
of
Mars
became
the
unifying
link
between
the
two
formerly
separate
realms
of
physics
and
cosmology.

It
may
be
objected
that
Kepler's
ideas
of
physics
were
so
primitive
that
they
ought
to
be
regarded
merely
as
a
subjective
stimulus
to
his
work
(like
the
five
perfect
solids),
without
objective
value.
In
fact,
however,
his
was
the
first
serious
attempt
at
explaining
the
mechanism
of
the
solar
system
in
terms
of
physical
forces,
and
once
the
example
was
set,
physics
and
cosmology
could
never
again
be
divorced.
And
secondly,
whereas
the
five
solids
were
indeed
merely
a
psychological
spur,
his
sky-physics
played,
as
we
saw,
a
direct
part
in
the
discovery
of
his
laws.

For,
although
the
functions
of
gravity
and
inertia
are
reversed
in
the
Keplerian
cosmos,
his
intuition
that
there
are
two
antagonistic
forces
acting
on
the
planets,
guided
him
in
the
right
direction.
A
single
force,
as
previously
assumed
(that
of
the
Prime
Mover
or
kindred
spirits)
could
never
produce
oval
orbits
and
periodic
changes
of
speed.
These
could
only
be
the
result
of
some
dynamic
tug-of-war
going
on
in
the
sky
–
as
indeed
they
are;
though
his
ideas
about
the
nature
of
the
"sun's
force"
and
the
planet's
"laziness"
or
"magnetism"
were
pre-Newtonian.

9.
The Pitfalls of Gravity

I
have
tried
to
show
that
without
his
invasion
into
the
territory
of
physics
Kepler
could
not
have
succeeded.
I
must
now
discuss
briefly
Kepler's
particular
brand
of
physics.
It
was,
as
to
be
expected,
physics-on-the-watershed,
half-way
between
Aristotle
and
Newton.
The
essential
concept
of
impetus
or
momentum,
which
makes
a
moving
body
persist
in
its
motion
without
the
help
of
an
external
force,
is
absent
from
it;
the
planets
must
still
be
dragged
through
the
ether
like
a
Greek
oxcart
through
the
mud.
In
this
respect
Kepler
had
not
advanced
further
than
Copernicus,
and
both
were
unaware
of
the
progress
made
by
the
Ockhamists
in
Paris.

On
the
other
hand,
he
came
very
near
to
discovering
universal
gravity,
and
the
reasons
for
his
failure
to
do
so
are
not
only
of
historical,
but
also
of
topical
interest.
Over
and
again
he
seems
to
balance
on
the
brink
of
the
idea
and
yet,
as
if
pulled
back
by
some
unconscious
resistance,
to
shrink
from
the
final
step.
One
of
the
most
striking
passages
is
to
be
found
in
the
introduction
to
the
Astronomia
Nova
.
There
Kepler
starts
by
demolishing
the
Aristotelian
doctrine
that
bodies
which
are
by
nature
"heavy"
strive
toward
the
centre
of
the
world,
whereas
those
which
are
"light"
strive
toward
its
periphery.
His
conclusions
are
as
follows: