DOC. 25 WHAT IS THE THEORY OF RELATIVITY 101
228
CONTRIBUTIONS TO SCIENCE
ing
the
war
in
the
land of
your
enemies. Even
though
the in-
vestigation
of the influence of the
gravitational
field of the
sun on
light
rays
is
a
purely objective
matter,
I
cannot
forbear
to
express my personal
thanks
to my English colleagues
for
their
work;
for without
it
I could
hardly
have lived
to
see
the
most
important implication
of
my
theory
tested.
[3]
We
can
distinguish
various kinds of theories in
physics.
Most
of them
are
constructive.
They attempt to
build
up
a
picture
of the
more complex
phenomena out
of the materials of
a
rela-
tively simple
formal scheme from which
they
start out.
Thus
the kinetic
theory
of
gases
seeks
to
reduce
mechanical,
thermal,
and diffusional
processes to
movements
of
molecules-i.e.,
to
build them
up out
of the
hypothesis
of molecular motion. When
we say
that
we
have succeeded in
understanding
a
group
of
natu-
ral
processes, we
invariably
mean
that
a
constructive
theory
has
been
found
which
covers
the
processes
in
question.
[4]
Along
with
this
most
important
class
of theories there exists
a
second,
which
I
will
call
"principle-theories."
These
employ
the
analytic,
not
the
synthetic,
method.
The
elements which
form
their
basis and
starting-point
are
not hypothetically con-
structed but
empirically
discovered
ones,
general
characteristics
of
natural
processes,
principles
that
give
rise
to mathematically
formulated criteria
which the
separate
processes or
the theoreti-
cal representations
of
them
have
to satisfy.
Thus the science of
thermodynamics
seeks
by analytical
means
to
deduce
necessary
conditions,
which
separate
events
have
to satisfy,
from the uni-
versally
experienced
fact
that
perpetual
motion
is
impossible.
The
advantages
of the constructive
theory
are
completeness,
adaptability,
and
clearness,
those of the
principle theory
are
logical
perfection
and
security
of the foundations.
The
theory
of
relativity belongs to
the latter
class.
In
order
to grasp
its
nature,
one
needs first
of all
to
become
acquainted
with the
principles
on
which it
is
based. Before
I go
into
these,
however,
I
must
observe that the
theory
of
relativity
resembles
a
building
consisting
of
two separate
stories,
the
special theory
and the
general theory.
The
special
theory,
on
which the
gen-
eral
theory
rests,
applies to
all
physical
phenomena
with
the