Albert Einstein	127

experience. The subjective time leads then through the concept of the bodily 
object and of space, to the concept of objective time, as we shall see later on.
  Ahead of the notion of objective time there is, however, the concept of 
space; and, ahead of the latter we find the concept of the bodily object. The 
latter is directly connected with complexes of sense experiences. It has been 
pointed out that one property which is characteristic of the notion "bodily 
object" is the property which provides that we coordinate to it an existence, 
independent of (subjective) time, and independent of the fact that it is 
perceived by our senses. We do this in spite of the fact that we perceive 
temporal alterations in it. Poincare has justly emphasized the fact that we 
distinguish two kinds of alterations of the bodily object, "changes of state" 
and "changes of position." The latter, he remarked, are alterations which we 
can reverse by arbitrary motions of our bodies.
  That there are bodily objects to which we have to ascribe, within a certain 
sphere of perception, no alteration of state, but only alterations of position, is 
a fact of fundamental importance for the formation of the concept of space 
(in a certain degree even for the justification of the notion of the bodily 
object itself). Let us call such an object "practically rigid."
  If, as the object of our perception, we consider simultaneously (i.e. as a 
single unit) two practically rigid bodies, then there exist for this ensemble 
such alterations as can not possibly be considered as changes of position of 
the whole, notwithstanding the fact that this is the case for each one of
the two constituents. This leads to the notion of "change of relative position" 
of the two objects; and, in this way, also to the notion of "relative
position" of the two objects. It is found moreover that among the relative 
positions, there is one of a specific kind which we designate as "Contact."* 
Permanent contact of two bodies in three or more "points" means that they 
are united as a quasi-rigid compound body. It is permissible to say that the 
second body forms then a (quasi-rigid) continuation on the first body and 
may, in its turn, be continued quasi-rigidly. The possibility of the quasi-rigid 
continuation of a body is unlimited. The real essence of the conceivable 
quasi-rigid continuation of a body Bo is the infinite
"space" determined by it.	.

· It is in the nature of things that we are able to talk about these objects only by means of
concepts of our own creation, concepts which themselves are not subject to definition. It is
essential, however, that we make use only of such concepts concerning whose coordination 
to
our experience we feel no doubt.