History is the only true teacher, the revolution the best school for the proletariat.

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

Marxism must abhor nothing so much as the possibility that it becomes congealed in its current form. It is at its best when butting heads in self-criticism, and in historical thunder and lightning, it retains its strength.

The more that social democracy develops, grows, and becomes stronger, the more the enlightened masses of workers will take their own destinies, the leadership of their movement, and the determination of its direction into their own hands.

Capitalism, as a result of its own inner contradictions, moves toward a point when it will be unbalanced, when it will simply become impossible.

Freedom for supporters of the government only, for members of one party only no matter how big its membership may be is no freedom at all. Freedom is always freedom for the man who thinks differently.

The masses are in reality their own leaders, dialectically creating their own development process.