Self-criticism, cruel, unsparing criticism that goes to the very root of the evil, is life and breath for the proletarian movement.

Those who do not move, do not notice their chains.

The working classes in every country only learn to fight in the course of their struggles.

Freedom is always and exclusively freedom for the one who thinks differently.

Social democracy.. is only the advance guard of the proletariat, a small piece of the total working masses; blood from their blood, and flesh from their flesh.

Only to the rude ear of one who is quite indifferent does the song of a bird seem always the same.

[Geology] opens up such wide intellectual vistas and supplies a more perfectly unified and more comprehensive conception of nature than any other science.