Physical concepts are free creations of the human mind, and are not,…

We shall, therefore, assume the complete physical equivalence of a gravitational field and a corresponding acceleration of the reference system.

Es ist bequem mit dem Einstein. Jedes Jahr widerruft er, was er das vorige Jahr geschrieben hat.

Everything should be made as simple as possible, but not simpler.

The mass of a body is a measure of its energy content.

By a clock we understand anything characterized by a phenomenon passing periodically through identical phases so that we must assume, by the principle of sufficient reason, that all that happens in a given period is identical with all that …

One reason why mathematics enjoys special esteem, above all other sciences, is that its laws are absolutely certain and indisputable, while those of other sciences are to some extent debatable and in constant danger of being overthrown by …