Sometimes one pays most for the things one gets for nothing

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

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

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

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 …

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

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 …