*Reference: Einstein’s 1920 Book*

*This paper presents Part 1, Chapter 1 from the book RELATIVITY: THE SPECIAL AND GENERAL THEORY by A. EINSTEIN. The contents are from the original publication of this book by Henry Holt and Company, New York (1920).*

*The paragraphs of the original
material (in black) are accompanied by brief comments (in color) based on the present
understanding. Feedback on these comments is appreciated.*

*The heading below is linked to the original materials.*

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**Physical Meaning of Geometrical Propositions**

In your schooldays most of you who read this book made acquaintance with the noble building of Euclid’s geometry, and you remember—perhaps with more respect than love—the magnificent structure, on the lofty staircase of which you were chased about for uncounted hours by conscientious teachers. By reason of your past experience, you would certainly regard every one with disdain who should pronounce even the most out-of-the-way proposition of this science to be untrue. But perhaps this feeling of proud certainty would leave you immediately if some one were to ask you: “What, then, do you mean by the assertion that these propositions are true?” Let us proceed to give this question a little consideration.

*Einstein
asks if the propositions of Euclid’s geometry are really true.*

Geometry sets out from certain conceptions such as “plane,” “point,” and “straight line,” with which we are able to associate more or less definite ideas, and from certain simple propositions (axioms) which, in virtue of these ideas, we are inclined to accept as “true.” Then, on the basis of a logical process, the justification of which we feel ourselves compelled to admit, all remaining propositions are shown to follow from those axioms, i.e. they are proven. A proposition is then correct (“true”) when it has been derived in the recognised manner from the axioms. The question of the “truth” of the individual geometrical propositions is thus reduced to one of the “truth” of the axioms. Now it has long been known that the last question is not only unanswerable by the methods of geometry, but that it is in itself entirely without meaning. We cannot ask whether it is true that only one straight line goes through two points. We can only say that Euclidean geometry deals with things called “straight line,” to each of which is ascribed the property of being uniquely determined by two points situated on it. The concept “true” does not tally with the assertions of pure geometry, because by the word “true” we are eventually in the habit of designating always the correspondence with a “real” object; geometry, however, is not concerned with the relation of the ideas involved in it to objects of experience, but only with the logical connection of these ideas among themselves.

*Einstein
finds that the propositions of Geometry have been formalized to conform to idealized
logic. They do not correspond with “real” objects. *

It is not difficult to understand why, in spite of this, we feel constrained to call the propositions of geometry “true.” Geometrical ideas correspond to more or less exact objects in nature, and these last are undoubtedly the exclusive cause of the genesis of those ideas. Geometry ought to refrain from such a course, in order to give to its structure the largest possible logical unity. The practice, for example, of seeing in a “distance” two marked positions on a practically rigid body is something which is lodged deeply in our habit of thought. We are accustomed further to regard three points as being situated on a straight line, if their apparent positions can be made to coincide for observation with one eye, under suitable choice of our place of observation.

If,
in pursuance of our habit of thought, we now supplement the propositions of
Euclidean geometry by the single proposition that two points on a practically
rigid body always correspond to the same distance (line-interval),
independently of any changes in position to which we may subject the body, the
propositions of Euclidean geometry then resolve themselves into propositions on
the possible relative position of practically rigid bodies.^{1}
Geometry which has been supplemented in this way is then to be treated as a
branch of physics. We can now legitimately ask as to the “truth” of geometrical
propositions interpreted in this way, since we are justified in asking whether
these propositions are satisfied for those real things we have associated with
the geometrical ideas. In less exact terms we can express this by saying that
by the “truth” of a geometrical proposition in this sense we understand its
validity for a construction with ruler and compasses.

^{1} It follows that a natural object is associated also with a
straight line. Three points A, B and C on a rigid body thus lie in a straight
line when, the points A and C being given, B is chosen such that the sum of the
distances AB and BC is as short as possible. This incomplete suggestion will
suffice for our present purpose.

*Geometrical propositions when made to correspond to real things may be treated as a branch of Physics.*

Of course the conviction of the “truth” of geometrical propositions in this sense is founded exclusively on rather incomplete experience. For the present we shall assume the “truth” of the geometrical propositions, then at a later stage (in the general theory of relativity) we shall see that this “truth” is limited, and we shall consider the extent of its limitation.

*The deeper meaning here is that geometry depends on the substance it describes. Einstein’s research on Light Quanta establishes light (radiation) as a substance in its own right. The geometry of light (radiation) shall be different from the geometry of material substance.*

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