*Reference: Einstein’s 1920 Book*

*This paper presents Appendix 2 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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## Minkowski’s Four-Dimensional Space (“World”)

Please see Appendix 2 at the link above.

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## FINAL COMMENTS

*The math of Minkowski’s World is based on the following consideration:*

*The fourth dimension is generally interpreted as the dimension
of time. But from the earlier comments it appears that it should be interpreted
as the dimension of the DURATION of substance. This is the dimension of inertia.*

*The first three dimensions are generally interpreted as the
dimensions of space. But it now appears that they should be interpreted as the
dimensions of the EXTENTS of substance. These are the dimensions of the substance
as quanta, which are taken as an abstract point for matter.*

*We may say that x, y, and z are the dimensions of what we have
been considering as a dimensionless point in space for matter.*

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