DOC. 71 PRINCETON LECTURES
PRE-RELATIVITY
PHYSICS
tion of
experience
to
our
concepts.
It
seems
to
me
that
Poincaré
clearly
recognized
the truth in the
account
he
[4]
gave
in his
book,
“La
Science et
l’Hypothese.” Among
all
the
changes
which
we
can
perceive
in
a
rigid body
those
which
can
be
cancelled
by
a
voluntary
motion
of
our
body
are
marked
by
their
simplicity;
Poincaré
calls
these,
changes
in
position. By
means
of
simple
changes
in
position
we can
bring two
bodies into
contact.
The theorems
of
con-
gruence,
fundamental
in
geometry,
have
to
do with the
laws
that
govern
such
changes
in
position.
For
the
con-
cept
of
space
the
following
seems
essential.
We
can
form
[5]
new
bodies
by bringing
bodies
B,
C,
...
up to body
A;
we say
that
we
continue
body
A.
We
can
continue
body A
in such
a way
that it
comes
into
contact
with
any
other
body,
X.
The
ensemble of all continuations of
body
A
we can
designate
as
the
“space
of the
body
A.” Then it
is true
that all bodies
are
in the
“space
of
the
(arbitrarily
chosen) body
A.”
In
this
sense we
cannot
speak
of
space
in
the
abstract,
but
only
of
the
“space belonging
to
a
body
A.”
The earth’s
crust
plays
such
a
dominant rôle
in
our
daily
life in
judging
the relative
positions
of
bodies
that
it
has
led
to
an
abstract
conception
of
space
which
certainly
cannot
be defended. In order
to
free
ourselves
from this
fatal
error we
shall
speak
only
of
“bodies
of
reference,”
or
“space
of
reference.” It
was
only through
the
theory
of
general relativity
that refinement
of these
concepts
became
necessary,
as we
shall
see
later.
I shall
not
go
into detail
concerning
those
properties
of
the
space
of reference which lead
to
our
conceiving
points as
elements
of
space,
and
space
as a
continuum.
Nor
shall I
attempt to analyse
further the
properties
of
space
which
justify
the
conception
of continuous
series
of
points,
or
lines.
If
these
concepts
are
assumed, together
[3]
263