D O C . 7 5 N O B E L L E C T U R E 7 5

merely a thing of thought. If we turn our focus to experimental physics, we see that

there the coordinate system always corresponds to a “practically rigid” body. It is

assumed, furthermore, that such rigid bodies allow themselves to be oriented rela-

tively at rest to one another, like bodies in Euclidean geometry. To the extent that

we may consider rigid measuring-bodies as objects of experience, as existing, it be-

comes feasible to tidy up the concept of a “coordinate system” as well as the con-

cept of the matter’s motion relative to it, in the sense of the “reality

postulate.”[3]

By way of this interpretation, Euclidean geometry also conforms at the same time

to the “reality postulate” in accordance with the needs of physics. The problem of

the validity of Euclidean geometry becomes physically meaningful; its validity is

presupposed in classical physics as well as later in the special theory of relativity.

The inertial frame and time in classical mechanics are best defined together by

a suitable formulation of the law of inertia: It is possible to determine time in such

a way and to assign to the coordinate system such a state of motion (inertial frame)

that, with reference to the latter, force-free material points undergo no acceleration;

it is additionally assumed that this time be concordantly measurable by identically

constructed clocks (periodic systems) in an arbitrary state of motion. Then, infinite-

ly many inertial frames exist that are in uniform translatory motion with respect to

each other; consequently, there are also infinitely many physically preferred states

of motion that are equivalent. Time is absolute, that is, independent of the choice

of a particular inertial frame; it is defined by more characteristics than are logically

necessary. However, as presupposed in mechanics, this should not give rise to con-

tradictions with experience. Let it be noted for now that, from the point of view of

the reality postulate, the logical weakness of this description is that experience does

not provide criteria for whether or not a material point is free of forces. That is why

the concept of an inertial frame remains somewhat problematic. This gap leads to

the general theory of relativity; let us leave it out of consideration for now.

In the consideration of the foundations of mechanics just outlined, the concept

of a rigid body (and the concept of a clock) plays a fundamental role that can be

challenged with a certain legitimacy. In nature, the rigid body is only approximate-

ly realized and not even to an arbitrary degree of approximation. Thus, this concept

does not strictly obey the “reality postulate.” Moreover, it seems logically unjusti-

fied to base all physical considerations on the rigid, that is, the solid body and then,

in the end, to reconstruct it atomistically again with the aid of the fundamental laws

of physics that are, in turn, constructed with the aid of the concept of a rigid mea-

suring-body. I mention these methodological deficiencies because, in the schematic

account presented here, they are also deficiencies of the theory of relativity to the

same extent. It would certainly be logically more correct to begin with the essence

[p. 3]